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Growth of thin films of Co3O4 by atomic layer deposition
Thin Solid Films 515 (2007) 7772 – 7781 www.elsevier.com/locate/tsf
Growth of thin films of Co3O4 by atomic layer deposition K.B. Klepper ⁎, O. Nilsen, H. Fjellvåg Centre for materials science and nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, Oslo N-0315, Norway Received 24 July 2006; received in revised form 24 January 2007; accepted 28 March 2007 Available online 11 April 2007
Abstract Thin films of cobalt oxide were made by atomic layer deposition (ALD), using Co(thd)2 (Hthd = 2,2,6,6-tetramethylheptan-3,5-dione) and ozone as precursors. Films were deposited on soda–lime glass and single crystals of Si(100). Pulse and purge parameters for ALD-type growth were established and such growth was found to occur for depositions within the temperature range of 114–307 °C. A preferred (100)-orientation was observed at the low end of the temperature range for films deposited on soda–lime glass and Si(100). At the high end of the temperature range, films deposited on Si(100) showed (111)-oriented growth, while films deposited on soda–lime glass substrates were unoriented. The electrical resistivity of as-deposited films on soda–lime glass were in the range of 0.13–4.48 Ω cm and showed a non-monotonic dependence on film thickness, with a minimum for films with a large proportion of grain boundaries. © 2007 Elsevier B.V. All rights reserved. Keywords: Cobalt oxide; Atomic layer deposition (ALD); Atomic layer epitaxy (ALE); Atomic layer chemical vapour deposition (ALCVD)
1. Introduction Atomic layer deposition (ALD, also known as atomic layer epitaxy and as atomic layer chemical vapour deposition) is a chemical gas phase thin film deposition technique based on alternating self-limiting gas-to-surface reactions [1,2]. The technique offers many advantages as a result of this growth mechanism, such as simple and accurate control of film thickness, even on substrates with intricate shapes, possibility to use solid precursors, easy scale up, elimination of gas phase reactions, and relatively low deposition temperatures. The disadvantage of relatively low growth rates is less important as the requirements for film thickness in nanotechnology are modest. There are two known stable oxides of cobalt; CoO and Co3O4. Of these, CoO adopts the rock salt type structure and Co3O4 the spinel type structure. CoO is thermodynamically stable at temperatures above 900 °C, however kinetically stable at room temperature. Co3O4 is formed at temperatures below 900 °C [3]. Thin films of CoO and Co3O4 have previously been
⁎ Corresponding author. Tel.: +47 22 85 55 75. E-mail address: [email protected] (K.B. Klepper). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.182
obtained by ALD [4,5], chemical vapour deposition [6–10], metal organic chemical vapour deposition (MOCVD) [11–17], sol–gel processes like dip coating [7,18–22] and spin coating [23,24], pulsed laser deposition [25], spray pyrolysis [26–28], electrodeposition [29], and sputtering [30]. Several types of precursors have been used depending on the method in question. For methods related to ALD, precursors like Co (acac)2 (Hacac = 2,4-pentanedione) [6,9,10,13,15–17], Co (acac)3 [12], Co(OAc)2 (HOAc = acetic acid) [8], CoI2 [5], and Co(thd)2 (Hthd = 2,2,6,6-tetramethylheptan-3,5-dione) have been used [7,11]. Of these, Co(acac)2 is by far the most common. In the present work Co(thd)2 and ozone were used. Cobalt oxides show electrochromic properties and are potential materials for non-emitting displays, smart windows, and thermal control for space vehicles [31–33]. They are attractive in solar cells as selective absorbers and corrosion protective coatings [12,29]. Cobalt is an important component in complex oxides like CoxFe3 − xO4 [34–36], LiCoO2 [37], LaCoO3 [4,38,39], and La1 − xSrxCoO3 [40], with potential applications like magnetostrictive torque sensors [35], magnetic recording media [36], spin valve pinning layers [34], electrochromic windows [37], rechargeable batteries [37], oxidation catalysts [40], gas sensors [38], as well as cathodes in high temperature ceramic fuel cells [39].
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2. Experimental details Thin films were deposited in a commercial F-120 Sat reactor (ASM Microchemistry) using ozone and Co(thd)2 as precursors. The Co(thd)2 was made in house as described in [41] and was sublimed at 109.5 °C during the depositions. Ozone was made by feeding O2 (99.999%, AGA) into an OT-020 generator from Ozone Technology, giving a specified ozone concentration of 15 vol.%. An ozone flow of ca. 500 cm3/min was used during the ozone pulses. During all depositions a background pressure of 1.8 mbar was obtained by applying a N2 carrier gas flow of 300 cm3 min− 1. The carrier gas was produced in a Schmidlin UHPN3001 N2 purifier with a claimed purity of 99.999% with regard to N2 + Ar content. The process was evaluated for socalled CVD-type growth by performing experiments at the specified conditions for Co(thd)2 sublimation and ozone flow, with one of the reactants not being pulsed. The films were deposited on substrates of soda–lime glass and Si(100) single crystals. Normally, one soda–lime glass and one Si(100) substrate were used in a vertical back-to-back configuration for a given run. The Si(100) substrates were used as obtained from the manufacturer, whereas the soda–lime glass substrates were cleaned according to the procedure described in [42]. Films were deposited using a total number of cycles ranging between 200 and 40 000, which resulted in thicknesses from ca. 4 to 940 nm. The films were examined with a Siemens D5000 X-ray diffractometer equipped with a Göbel-mirror which provides parallel Cu Kα radiation. The setup was used to measure X-ray
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reflectivity (XRR), conventional X-ray diffraction (XRD) in reflection mode, as well as rocking curve analysis (ω-scans) and φ-analysis. X-ray fluorescence (XRF) measurements were performed on a Philips PW2400 for an alternative assessment of film thickness. The XRF-intensities were analyzed using the UniQuant software [43]. An alternative and fast assessment of the film thickness was obtained using an ordinary document scanner in transmittance setup (ordinarily used to scan photo negatives). This method was used for films deposited on transparent soda–lime glass substrates. The film thickness was calculated by using Beer's law on the white light absorption of the Co3O4 film. The method is simple and gives an uncertainty of ca. 10%. A somewhat higher precision might be obtained by using the film intensity interpretations by Macleod [44], however inclusion of more material parameters are then necessary. CoO and Co3O4 are both semiconductors. The electrical resistivity of obtained films was studied as function of film thickness and deposition temperature. The resistivity was measured by a linear four-point probe setup with 1 mm spacing between the probes, and probe tips with a curvature of 0.5 mm. Selected films were studied with atomic force microscopy (AFM; Dimension 3100 with Nanoscope IIIa controller) in order to determine topography, number of surface terminating crystals and roughness. Selected films were analysed with transmission Fourier transform infrared (FTIR) spectroscopy on a Perkin–Elmer System 2000 FTIR spectrometer. The studied films were deposited on both sides of Si(100) substrates (both sides polished). An uncoated substrate was used as reference.
Fig. 1. Growth rate of thin films of Co3O4 deposited at 186 °C as function of duration of pulse and purge times of Co(thd)2 (a, c) and O3 (b, d). Filled squares refers to Si (100) substrates and open triangles to soda–lime glass substrates. All measurements refer to the middle of the substrate (3.1 cm from first impact of the precursor on soda–lime glass and 2.3 cm on Si(100).
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Fig. 2. Growth rate as function of deposition temperature for Co3O4 films deposited during 1000 cycles on (a) soda–lime glass substrates and (b) Si(100). The film thicknesses in (a) at the points A, B, C and D, indicated in the inset, are measured by scanning, and XRR at point X is used for calibration of the scanner. The film thicknesses in (b) at the points F, M and X in the inset, are measured by XRR.
Fig. 4. Film thickness as function of number of deposition cycles on (a) soda– lime glass and (b) Si(100) when deposited at 186 °C. Thicknesses measured by XRF are given by stars. The film thicknesses on soda–lime glass are measured by scanning at the points A, B, C and D as indicated in the inset (a). Point X is measured by XRR for calibration of the scanner. The inset in (b) shows the measured points with XRR on Si(100) substrates.
Pelletized powder samples of Co3O4 and CoCO3 in a matrix of KBr were also measured by FTIR spectroscopy, using a blank pellet as reference.
keeping all others at values well within the ALD-growth regime (Fig. 1). The results show that a 1.5 s pulse of Co(thd)2 and a 6.0 s pulse of ozone is sufficient for ALD-type growth. The subsequent purging was kept at 1.0 s after a Co(thd)2 pulse and 1.5 s after an ozone pulse. It is apparent from Fig. 1 that even shorter purge times are sufficient to prevent CVD type growth. The growth rate was measured with XRR and scanning (Fig. 2). For both types of substrates, the growth rate varied only slightly with the temperature between 114 and 307 °C. The growth rate increased abruptly above 307 °C, indicating uncontrolled growth. The thermal decomposition temperature of the precursor was determined to be 310 ± 6 °C, according to the method described in Ref. [45]. Hence, the uncontrolled growth is likely to be a result of decomposition of Co(thd)2. The measured decomposition temperature is almost 100 °C lower than what was reported in Ref. [4]. The thicknesses of films deposited at 332 °C (Fig. 2) were estimated from optical scanning on soda–lime glass. Representative thickness gradients for depositions at 186 °C on soda–lime glass substrates are given in Fig. 3. It is apparent that the film thickness decreases with distance from the point of first impact between precursor and substrate. The decrease appears both along the horizontal and vertical flow directions. This probably reflects that the precursor is introduced to the reaction chamber from a point source rather than from a line source. The decrease in thickness within 2 cm along the flow direction from the leading end of the substrates is 2.6 and 9.3% for Si(100) and soda–lime glass, respectively. The Si(100)
3. Results The influence of pulse and purge parameters on the growth was investigated by changing one parameter at a time, and
Fig. 3. Thickness gradients for a Co3O4 film deposited at 186 °C during 1000 cycles along the length of the substrate, simulated using an optical scanned film. The thickness of the film is indicated in nm on the contour lines.
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Fig. 5. θ–2θ XRD data for films deposited on (a) soda–lime glass and (b) Si(100) as function of deposition temperature (138–283 °C).
substrates were measured by XRR, whilst soda–lime glass substrates were measured by optical scanning. Note that the uncertainty for the latter (∼ 10%) is much larger than for XRR (∼ 1%). The film thickness as function of deposition cycles was investigated in detail for a reaction temperature of 186 °C; pulse and purge times of 1.5 s pulse of Co(thd)2, 1.0 s purge with N2, 6.0 s ozone pulse, followed by a 1.5 s purge with N2. The film thicknesses were measured by XRR, optical scanning (calibrated with XRR), and XRF. The thickness and gradients at 200 cycles were measured by XRR, as these were below the detection limit of the optical scanner. The average thickness at 5000 cycles was measured by XRF, since this exceeds the upper limit for XRR. Fig. 4 reveals a linear relationship between the film thickness and the number of cycles for films deposited on soda–lime glass as well as on Si(100). The average growth rate on soda–lime glass is 0.022 ± 0.002 and 0.025 ± 0.003 nm/cycle as measured by XRR and XRF, respectively. For films deposited on Si(100) substrates, the corresponding rates are 0.020 ± 0.001 and 0.024 ± 0.002 nm/cycle, respectively. Representative XRD patterns are shown in Fig. 5. All films proved to be phase-pure Co3O4. However, the growth orientation was dependent on the deposition temperature and the type of substrate. For the lower part of the applicable 114 – 307 °C deposition range, the films show a tendency towards (100)-oriented growth on both types of substrates. At the high temperature end of this range, films
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Fig. 6. (a) FTIR spectra of films deposited in the range of 138–283 °C. Panel (b) shows spectra of powder samples of CoCO3 and Co3O4 for comparison.
deposited on Si(100) substrates show a (111)-oriented growth whereas films on soda-lime glass loose their preferred (100)orientation for depositions at 283 °C. A comparison of FTIR spectra of powder samples of Co3O4 and CoCO3 with that of Co3O4 films is given in Fig. 6. It is first noted that the FTIR spectra do not indicate presence of carbonate in the deposited films. Second, although the FTIR
Fig. 7. Roughness as function of number of deposition cycles on soda–lime glass (open squares) and Si(100) (filled squares) when deposited at 186 °C. Inset; Roughness as function of deposition temperature on soda–lime glass (open squares) and Si(100) (filled squares) when deposited with 1000 cycles.
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Fig. 8. AFM topography images for increasing number of deposition cycles for Co3O4 films on Si(100) and soda–lime glass when deposited at 186 °C.
spectrum for powder Co3O4 is very similar to those of the deposited films, the two main absorption peaks at 580 and 667 cm− 1 for Co3O4 powder are shifted slightly to 556 and 657 cm− 1 for the film. This shift towards lower wave numbers may be a result of the limited film thickness [46]. The variation of surface roughness as function of deposition cycles on soda–lime glass and Si(100), as measured by AFM, is shown in Fig. 7. For films deposited using 1000 cycles within the temperature range 114 – 283 °C, the average root mean square (RMS) roughness is 1.16 and 1.27 nm for depositions on soda–lime glass and Si(100), respectively, see inset in Fig. 7. The roughness is much larger for films deposited at 307 °C or above. The AFM topographies for some selected films are shown in Fig. 8. The size of the surface terminating crystals increases with the number of deposition cycles for both types of substrate. Their average diameter increases from 21 to 50 nm for film
deposited within 1000 to 40 000 cycles on soda–lime glass. Correspondingly, the average diameter increases from 22 to 62 nm for films deposited on Si(100) within 1000 to 40 000 cycles. The number of crystals at the terminating surface was obtained from AFM images as a function of deposition cycles, Fig. 9a. The results from the counting procedure have shown to be dependent on the quality of the AFM images, where a good quality will give a larger number of terminating crystals. The datasets in Fig. 9 are based on images with similar quality except for the measuring point at 10 000 cycles, which had a remarkable good quality. This point may thus be systematically larger than the others. The general trend, visualized by the fully drawn line, indicates a decrease in the number of crystals with increasing number of deposition cycles. The trend as drawn in Fig. 9a has a similar shape to that reported for simulations of comparable situations, Ref. [47]. The size of the crystals
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changes in the microstructure, as well as with the decomposition of the Co(thd)2 precursor at temperatures above 307 °C. The specific resitivity at room temperature for films deposited on soda–lime glass was evaluated as function of film thickness. Measurements were done at four positions along the flow direction, see inset in Fig. 11b. The thickness at the measuring positions was derived by optical scanning (calibrated by XRR and/or XRF). The thickness of the film deposited with 10 000 cycles was measured by XRF to be 241 ± 4 nm giving a specific resistivity of 5.9 ± 0.1 Ω cm, while that of 40 000 cycles was estimated (extrapolation of linear type growth) to be 940 ± 89 nm giving 80 ± 1 Ω cm. These results display a trend where the specific resistivity at room temperature versus film thickness goes through a minimum for a film thickness of some 40 nm before increasing with film thickness, which is surprising. 4. Discussion
Fig. 9. Number of surface terminating crystals for an area of 1 μm2 as function of (a) number of deposition cycles at 186 °C, and (b) function of temperature.
increase with increasing deposition temperature for a given number of cycles (Fig. 10), but the effect is less pronounced than what was found with respect to crystal size versus number of cycles, see Figs. 8 and 10. The number of crystals as a function of deposition temperature is given in Fig. 9b. Rocking curve analysis of films deposited on soda–lime glass was generally performed for the strongest reflection in each pattern shown in Fig. 5. The results are summarized in Table 1. These again emphasize that films deposited on soda– lime glass have a certain preferred (100)-orientation in the deposition range of 186–235 °C, whereas the film deposited at 138 °C shows a more random orientation. At 283 °C the preferred orientation seems to be lost. Films of Co3O4 deposited on Si(100) at 138 and 186 °C show the same tendency towards a preferred (100)-orientation. However, the preferred orientation is lost already at 235 °C. At 283 °C, the full width at half maximum (FWHM) decreases with ∼ 50% and the reflections become consistent with a (111)-oriented film. This is somewhat surprising as the surface of Si(100) is considered to be covered with an amorphous SiOx layer. However, φ-analysis of the film showed no in-plane orientation of the crystals. Specific resistivity at room temperature for films deposited on soda–lime glass is shown as function of deposition temperature in Fig. 11a. The specific resistivity increases from 0.14 to 1.12 Ω cm over the temperature range 114 – 259 °C, thereafter it increases significantly, reaching 3.27 Ω cm at 307 °C and 9.39 Ω cm at 332 °C. This increase correlates with
A possible mechanism for the thickness gradients as presented in Fig. 3 may be connected to the limited life time of ozone, which decomposes quite rapidly into O2 in the relevant temperature range. The half-life of ozone at 250 °C is only 0.03 s [48] and will be even shorter if collisions with other molecules are taken into account. This suggests that ozone will not diffuse uniformly within the whole reaction chamber. In addition, the film itself may catalyze decomposition of ozone, as is well known for MnO2 [48]. In this case, ozone is not likely to survive collisions with any surfaces in the reaction chamber. This mechanism is supported by the fact that we need several times the ozone flux to produce cobalt oxide films as what was necessary to produce calcium carbonate films with the same equipment [49]. The out-of-plane orientation for films deposited on Si(100) at 283 °C, and the lack of such orientation on soda-lime glass at the same deposition temperature, was surprising. The Si(100) substrates used in the current experiments are terminated by a layer of native amorphous SiOx. Hence, both types of substrates have an amorphous surface that should not impose oriented film growth. The difference in film orientation becomes apparent only at higher temperatures. This may indicate that the morphology of the crystals forming the film is altered from octahedrons to cubes as the temperature is increased [47]. This effect should, however, be substrate independent. The native amorphous SiOx layer on Si(100) is commonly in the range of 0.5–2 nm. It may thus be imagined that cobalt can diffuse through this layer and form arrangements that guide the film growth. However, we find it questionable that a Si(100) surface, which has fourfold symmetry, should direct the growth of the cobalt oxide into a (111) orientation, which has threefold symmetry. The FWHM data in Table 1 indicate that the tilt between the crystals is large regardless of temperature, with exception of the film deposited on Si(100) at 283 °C. Rocking curve analysis was similarly performed for the reflection with the highest intensity for the individual films deposited at 186 °C. The results are presented as function of deposition cycles (and film thickness) in Table 2. At these
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Fig. 10. AFM topography images of Co3O4 films on Si(100) and soda–lime glass substrates as function of increasing deposition temperature.
conditions (100)-oriented growth is achieved. The FWHM decrease slightly with increasing number of cycles on both types of substrates, with exception of the film deposited with 40 000 cycles. For the latter it was required to stop the deposition every 10 000 cycles to refill with precursor, which may be a cause for the increased FWHM. The preferred (100)-orientation at low temperatures can be explained if the crystallites take the morphology of octahedra. The surface of the films will then be terminated by {111} surfaces of Co3O4. The fastest growth direction would be along b100N which results in (100)-oriented films, as observed (cf. Ref. [47]). As the adopted AFM tips were spherical with a size comparable to that of the crystals (tip curvature radius of ∼ 20 nm), the proposed octahedron shape of the surface terminating crystals will be convoluted and appear as spheres in topography images. On the other hand, if the crystals really were shaped as spheres, a very low surface roughness should
have resulted (cf. Ref. [50]), which is not observed. It is thus likely that the surface terminating crystals have a specific shape, probably of an octahedron owing to the observed (100)preferred growth orientation at temperatures below 235 °C. If the surface terminating crystals kept the octahedron morphology for the (111)-oriented films, it would imply that the {111} surfaces of the octahedron were parallel with the substrate surface, leading to very low roughness. However, the observed roughness is significant, i.e. 1.94 nm (RMS). It is more consistent to assume that the surface terminating crystals grow as cubes, with their corners pointing outwards from the surface as in (111)oriented films, producing a notable surface roughness, see the inset in Fig. 7. An AFM image obtained for a film deposited at 259 °C on Si(100), shows triangular shaped crystals which are consistent with the corners of a cube oriented in the [111] direction, Fig. 12. The orientation of the triangular shaped crystals does not show any in-plane orientation which is consistent with
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Table 1 FWHM of XRD reflections for Co3O4 on substrates of soda–lime glass and Si(100)
Table 2 FWHM of XRD reflections for films of Co3O4 on substrates of soda–lime glass and Si(100)
Temperature (°C)
FWHM on soda–lime glass (°)
FWHM on Si(100) (°)
Number of cycles
138 186 235 283
14 (400) 12 (400) 14 (400) 18 (311)
14 (400) 13 (400) 13 (311) 7 (111)
FWHM on soda–lime glass (°)
FWHM on Si(100) (°)
3000 (∼70 nm) (311) 5000 (∼125 nm) (400) 10 000 (∼ 240 nm) (400) 40 000 (∼ 960 nm) (400)
22 19 17 21
19 18 15 21
Miller indices are given in parentheses.
Miller indices are given in parentheses.
the XRD φ-analysis. The lack of in-plane orientation also undermines a potential claim of a misshaped AFM tip. Growth of (111)-oriented Co3O4 from formation of cube shaped crystallites, has been reported earlier on substrates of Si (100) and/or glass [11,12,15,17]. In both cases the films were grown by MOCVD, using respectively Co(acac)3 (380–500 °C) and Co(acac)2 (375–550 °C) as precursors, notably at relatively high temperatures. Assuming that there is a conversion in growth regime from octahedron to cube morphology as the deposition temperature is increased, one would anticipate a minimum in surface roughness at the conversion temperature (see Ref. [47]). A small, yet detectable, minimum in surface roughness is observable for depositions on Si(100) around 250 °C, see inset in Fig. 7.
Fig. 11. Specific resistivity of Co3O4 films on soda–lime glass at room temperature as: (a) Function of deposition temperature for films deposited with 1000 cycles. (b) Function of film thickness for films deposited at 186 °C. Measuring points are indicated on the insets.
As evident from Fig. 8, the Co3O4 films deposited at 186 °C, with a preferred (100)-orientation, show an increasing size of the surface terminating crystals with increasing number of deposition cycles. This behaviour is typical for columnar growth. Amorphous surfaces, like soda–lime glass and in principle also Si(100), will initially orient the crystals randomly. Some of the crystals will have their highest growth rate in a direction normal to the substrate. These will in the end enclose the others completely (see Ref. [47]). As a result, the number of surface terminating crystals decreases, and the film surface will be dominated by crystals with their fastest growth direction normal to the substrate. As seen from Fig. 9b, the number of surface terminating crystals decreases with increasing temperature. The spread in tilt of the crystals also decreases slightly with increasing temperature. These observations are reasonable since the mobility of the adsorbed precursor fragments should increase with temperature, and hence also the tendency towards forming dense layers matching the crystallographic lattice of the underlying crystal. This will govern certain crystallographic surface terminations and more defined crystal habits will be developed. It might be intuitive that the film roughness should increase monotonically with increasing film thickness, however, as Fig. 7 shows, the film roughness does not follow this pattern.
Fig. 12. AFM topography image of Co3O4 deposited at 259 °C on a Si(100) substrate by 1000 cycles.
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The roughness increases towards a maximum for ca. 2500 cycles, before it goes through a local minimum and then appears to reach an equilibrium value. Care was taken during the measurements to prevent instrumental variations to have an impact on the results. The increase in specific resistivitiy as function of film thickness above some 40 nm is surprising (Fig. 11b). Normally, one would expect the resistivity to decrease with thickness due to better crystallinity. When comparing Figs. 7 and 11b it becomes evident that the observed minimum in film resistivity occurs in the same area where there is a maximum in surface roughness, indicating that the conduction is greatly affected by the microstructure of the film. According to the simulations in Ref. [47] the microstructure at this minimum should be of a fully covering film with a large proportion of grain-boundaries, which may indicate that the conduction paths are along the grain boundaries and not transverse the grains. The present results contradict those obtained by Kadam and Patil [26] for Co3O4 films deposited by spray pyrolysis. In addition to observing a decrease in specific resistance with increasing thickness, they found a much larger specific resistance (2.1 × 104 Ω cm for a 900 nm film) than for the present case (80 Ω cm, ∼ 940 nm). Similarly, a high specific resistivity of 316 Ω cm has been reported for 100 nm Co3O4 films deposited by spin coating [24]. The presently observed high resistivity for very thin films (200 cycles corresponding to 4 nm) is according to expectations since these consists of small crystallites with large relative thickness variations and may perhaps not be fully interconnected on the surface. 5. Conclusion Thin films of Co3O4 were made using Co(thd)2 (Hthd = 2,2,6,6-tetramethylheptan-3,5-dione) and ozone as precursors. Substrates of soda–lime glass and single crystals of Si(100) were used and ALD-type growth of Co3O4 was established in the temperature range 114–307 °C. All films are crystalline and with a preferred (100)-orientation at the low end of the temperature range, independent of the type of substrate. At the high end of the temperature range, films deposited on Si(100) showed (111)oriented growth, while films deposited on soda–lime glass substrates were unoriented. The orientation preferences are most likely accompanied by an octahedron-shaped morphology at low temperatures, and a cube-shaped morphology for films deposited on Si(100) at high temperatures. The measured specific resistivity on soda–lime glass substrates showed a non-monotonic dependence on film thickness with a minimum for films with a high proportion of grain boundaries, and surprisingly increased with increasing thickness. Acknowledgments This work has received financial support from the Department of Chemistry, University of Oslo, Norway. The authors are indebted to Oddvar Dyrlie for help and discussions concerning the AFM measurements, and to Cand. Scient. Martin Lie for XRF measurements.
References [1] M. Ritala, M. Leskelä, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, vol. 1, Academic Press, San Diego CA, 2002, p. 103. [2] R.L. Puurunen, Appl. Phys. 97 (2005) 121301. [3] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, ButterworthHeinemann, Amsterdam, 1997, p. 1113. [4] H. Seim, M. Nieminen, L. Niinisto, H. Fjellvåg, L.S. Johansson, Appl. Surf. Sci. 112 (1997) 243. [5] M. Rooth, E. Lindahl, A. Harsta, Chem. Vap. Depos. 12 (2006) 209. [6] L.M. Apatiga, V.M. Castano, Thin Solid Films 496 (2006) 576. [7] L. Armelao, D. Barreca, S. Gross, E. Tondello, in: D. Davazoglou, C. Vahlas (Eds.), Thirteenth European Conference on Chemical Vapor Deposition, Athens, Greece, August 26–31, 2001, J. Phys. IV France: Proc., vol. 11, 2001, p. 437. [8] T. Maruyama, T. Nakai, Sol. Energy Mater. 23 (1991) 25. [9] E.F. Rivera, B. Atakan, K. Kohse-Hoinghaus, in: D. Davazoglou, C. Vahlas (Eds.), Thirteenth European Conference on Chemical Vapor Deposition, Athens, Greece, August 26–31, 2001, J. Phys. IV France: Proc., vol. 11, 2001, p. 629. [10] K. Shalini, A.U. Mane, S.A. Shivashankar, M. Rajeswari, S. Choopun, J. Cryst. Growth 231 (2001) 242. [11] M. Burriel, G. Garcia, J. Santiso, A. Abrutis, Z. Saltyte, A. Figueras, Chem. Vap. Depos. 11 (2005) 106. [12] M. Burriel, G. Garcia, J. Santiso, A.N. Hansson, S. Linderoth, A. Figueras, Thin Solid Films 473 (2005) 98. [13] E. Fujii, H. Torii, A. Tomozawa, R. Takayama, T. Hirao, J. Mater. Sci. 30 (1995) 6013. [14] A. Gulino, I. Fragala, Inorg. Chim. Acta 358 (2005) 4466. [15] A.U. Mane, K. Shalini, S.A. Shivashankar, in: D. Davazoglou, C. Vahlas (Eds.), Thirteenth European Conference on Chemical Vapor Deposition, Athens, Greece, August 26–31, 2001, Journal de Physique IV: Proceedings, vol. 11, 2001, p. 637. [16] A.U. Mane, K. Shalini, A. Wohlfart, A. Devi, S.A. Shivashankar, J. Cryst. Growth 240 (2002) 157. [17] A.U. Mane, S.A. Shivashankar, J. Cryst. Growth 254 (2003) 368. [18] E. Barrera, T. Viveros, A. Avila, P. Quintana, M. Morales, N. Batina, Thin Solid Films 346 (1999) 138. [19] F.C. Eze, Semicond. Sci. Technol. 16 (2001) 362. [20] B. Pejova, A. Isahi, M. Najdoski, I. Grozdanov, Mater. Res. Bull. 36 (2001) 161. [21] M.S. Selim, J. Cryst. Growth 265 (2004) 115. [22] H. Tototzintle-Huitle, E. Prokhorov, A. Mendoza-Galvan, J.E. Urbina, J. Gonzalez-Hernandez, J. Phys. Chem. Solids 64 (2003) 975. [23] M. Sato, H. Hara, H. Kuritani, T. Nishide, Sol. Energy Mater. Sol. Cells 45 (1997) 43. [24] C.F. Windisch Jr., K.F. Ferris, G.J. Exarhos, J. Vac. Sci. Technol. A 19 (2001) 1647. [25] R.J. Kennedy, IEEE Trans. Magn. 31 (1995) 3829. [26] L.D. Kadam, P.S. Patil, Mater. Chem. Phys. 68 (2001) 225. [27] P.S. Patil, L.D. Kadam, C.D. Lokhande, Thin Solid Films 272 (1996) 29. [28] V.R. Shinde, S.B. Mahadik, T.P. Gujar, C.D. Lokhande, Appl. Surf. Sci. 252 (2006) 7487. [29] G. McDonald, Thin Solid Films 72 (1980) 83. [30] J.G. Cook, M.P. Van der Meer, Thin Solid Films 144 (1986) 165. [31] G.C. de Vries, Electrochim. Acta 44 (1999) 3185. [32] C.G. Granqvist, E. Avendano, A. Azens, Thin Solid Films 442 (2003) 201. [33] J.G.H. Mathew, S.P. Sapers, M.J. Cumbo, N.A. O'Brien, R.B. Sargent, V.P. Raksha, R.B. Lahaderne, B.P. Hichwa, J. Non-Cryst. Solids 218 (1997) 342. [34] M.J. Carey, S. Maat, P. Rice, R.F.C. Farrow, R.F. Marks, A. Kellock, P. Nguyen, B.A. Gurney, Appl. Phys. Lett. 81 (2002) 1044. [35] Y. Chen, J.E. Snyder, C.R. Schwichtenberg, K.W. Dennis, R.W. McCallum, D.C. Jiles, IEEE Trans. Magn. 35 (1999) 3652. [36] Y. Kitamoto, S. Kantake, F. Shirasaki, M. Abe, M. Naoe, J. Magn. Magn. Mater. 193 (1999) 97. [37] R.B. Goldner, F.O. Arntz, T.E. Haas, Sol. Energy Mater. Sol. Cells 32 (1994) 421.
K.B. Klepper et al. / Thin Solid Films 515 (2007) 7772–7781 [38] T. Arakawa, in: L.G. Tejuca, J.L.G. Fierro (Eds.), Properties and Applications of Perovskite-Type Oxides, Marcel Dekker Inc., New York NY, 1993, p. 361. [39] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [40] T. Nakamura, M. Misono, Y. Yoneda, J. Catal. 83 (1983) 151. [41] G.S. Hammond, D.C. Nonhebel, C.H.S. Wu, Inorg. Chem. 2 (1963) 73. [42] O. Nilsen, H. Fjellvag, A. Kjekshus, Thin Solid Films 444 (2003) 44. [43] UniQuant Version 2 User Manual, Omega Data Systems, Neptunus 2, NL5505 Veldhoven, The Netherlands, 1994. [44] H.A. Macleod, Thin-film optical filters, Institute of Physics Publishing, Bristol UK, 2001.
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[45] O. Nilsen, H. Fjellvåg, A. Kjekshus, Thermochim. Acta 404 (2003) 187. [46] V.P. Tolstoj, I.V. Cernysova, V.A. Skrysevskij, Handbook of infrared spectroscopy of ultrathin films, Wiley-Interscience, Hoboken N.J., 2003. [47] O. Nilsen, O.B. Karlsen, A. Kjekshus, H. Fjellvåg, Thin Solid Films 515 (2006) 4527. [48] S.T. Oyama, Catal. Rev., Sci. Eng. 42 (2000) 279. [49] O. Nilsen, H. Fjellvåg, A. Kjekshus, Thin Solid Films 450 (2004) 240. [50] O. Nilsen, O.B. Karlsen, A. Kjekshus, H. Fjellvåg, Thin Solid Films 515 (2006) 4538.
Growth of thin films of Co3O4 by atomic layer deposition K.B. Klepper ⁎, O. Nilsen, H. Fjellvåg Centre for materials science and nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, Oslo N-0315, Norway Received 24 July 2006; received in revised form 24 January 2007; accepted 28 March 2007 Available online 11 April 2007
Abstract Thin films of cobalt oxide were made by atomic layer deposition (ALD), using Co(thd)2 (Hthd = 2,2,6,6-tetramethylheptan-3,5-dione) and ozone as precursors. Films were deposited on soda–lime glass and single crystals of Si(100). Pulse and purge parameters for ALD-type growth were established and such growth was found to occur for depositions within the temperature range of 114–307 °C. A preferred (100)-orientation was observed at the low end of the temperature range for films deposited on soda–lime glass and Si(100). At the high end of the temperature range, films deposited on Si(100) showed (111)-oriented growth, while films deposited on soda–lime glass substrates were unoriented. The electrical resistivity of as-deposited films on soda–lime glass were in the range of 0.13–4.48 Ω cm and showed a non-monotonic dependence on film thickness, with a minimum for films with a large proportion of grain boundaries. © 2007 Elsevier B.V. All rights reserved. Keywords: Cobalt oxide; Atomic layer deposition (ALD); Atomic layer epitaxy (ALE); Atomic layer chemical vapour deposition (ALCVD)
1. Introduction Atomic layer deposition (ALD, also known as atomic layer epitaxy and as atomic layer chemical vapour deposition) is a chemical gas phase thin film deposition technique based on alternating self-limiting gas-to-surface reactions [1,2]. The technique offers many advantages as a result of this growth mechanism, such as simple and accurate control of film thickness, even on substrates with intricate shapes, possibility to use solid precursors, easy scale up, elimination of gas phase reactions, and relatively low deposition temperatures. The disadvantage of relatively low growth rates is less important as the requirements for film thickness in nanotechnology are modest. There are two known stable oxides of cobalt; CoO and Co3O4. Of these, CoO adopts the rock salt type structure and Co3O4 the spinel type structure. CoO is thermodynamically stable at temperatures above 900 °C, however kinetically stable at room temperature. Co3O4 is formed at temperatures below 900 °C [3]. Thin films of CoO and Co3O4 have previously been
⁎ Corresponding author. Tel.: +47 22 85 55 75. E-mail address: [email protected] (K.B. Klepper). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.182
obtained by ALD [4,5], chemical vapour deposition [6–10], metal organic chemical vapour deposition (MOCVD) [11–17], sol–gel processes like dip coating [7,18–22] and spin coating [23,24], pulsed laser deposition [25], spray pyrolysis [26–28], electrodeposition [29], and sputtering [30]. Several types of precursors have been used depending on the method in question. For methods related to ALD, precursors like Co (acac)2 (Hacac = 2,4-pentanedione) [6,9,10,13,15–17], Co (acac)3 [12], Co(OAc)2 (HOAc = acetic acid) [8], CoI2 [5], and Co(thd)2 (Hthd = 2,2,6,6-tetramethylheptan-3,5-dione) have been used [7,11]. Of these, Co(acac)2 is by far the most common. In the present work Co(thd)2 and ozone were used. Cobalt oxides show electrochromic properties and are potential materials for non-emitting displays, smart windows, and thermal control for space vehicles [31–33]. They are attractive in solar cells as selective absorbers and corrosion protective coatings [12,29]. Cobalt is an important component in complex oxides like CoxFe3 − xO4 [34–36], LiCoO2 [37], LaCoO3 [4,38,39], and La1 − xSrxCoO3 [40], with potential applications like magnetostrictive torque sensors [35], magnetic recording media [36], spin valve pinning layers [34], electrochromic windows [37], rechargeable batteries [37], oxidation catalysts [40], gas sensors [38], as well as cathodes in high temperature ceramic fuel cells [39].
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2. Experimental details Thin films were deposited in a commercial F-120 Sat reactor (ASM Microchemistry) using ozone and Co(thd)2 as precursors. The Co(thd)2 was made in house as described in [41] and was sublimed at 109.5 °C during the depositions. Ozone was made by feeding O2 (99.999%, AGA) into an OT-020 generator from Ozone Technology, giving a specified ozone concentration of 15 vol.%. An ozone flow of ca. 500 cm3/min was used during the ozone pulses. During all depositions a background pressure of 1.8 mbar was obtained by applying a N2 carrier gas flow of 300 cm3 min− 1. The carrier gas was produced in a Schmidlin UHPN3001 N2 purifier with a claimed purity of 99.999% with regard to N2 + Ar content. The process was evaluated for socalled CVD-type growth by performing experiments at the specified conditions for Co(thd)2 sublimation and ozone flow, with one of the reactants not being pulsed. The films were deposited on substrates of soda–lime glass and Si(100) single crystals. Normally, one soda–lime glass and one Si(100) substrate were used in a vertical back-to-back configuration for a given run. The Si(100) substrates were used as obtained from the manufacturer, whereas the soda–lime glass substrates were cleaned according to the procedure described in [42]. Films were deposited using a total number of cycles ranging between 200 and 40 000, which resulted in thicknesses from ca. 4 to 940 nm. The films were examined with a Siemens D5000 X-ray diffractometer equipped with a Göbel-mirror which provides parallel Cu Kα radiation. The setup was used to measure X-ray
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reflectivity (XRR), conventional X-ray diffraction (XRD) in reflection mode, as well as rocking curve analysis (ω-scans) and φ-analysis. X-ray fluorescence (XRF) measurements were performed on a Philips PW2400 for an alternative assessment of film thickness. The XRF-intensities were analyzed using the UniQuant software [43]. An alternative and fast assessment of the film thickness was obtained using an ordinary document scanner in transmittance setup (ordinarily used to scan photo negatives). This method was used for films deposited on transparent soda–lime glass substrates. The film thickness was calculated by using Beer's law on the white light absorption of the Co3O4 film. The method is simple and gives an uncertainty of ca. 10%. A somewhat higher precision might be obtained by using the film intensity interpretations by Macleod [44], however inclusion of more material parameters are then necessary. CoO and Co3O4 are both semiconductors. The electrical resistivity of obtained films was studied as function of film thickness and deposition temperature. The resistivity was measured by a linear four-point probe setup with 1 mm spacing between the probes, and probe tips with a curvature of 0.5 mm. Selected films were studied with atomic force microscopy (AFM; Dimension 3100 with Nanoscope IIIa controller) in order to determine topography, number of surface terminating crystals and roughness. Selected films were analysed with transmission Fourier transform infrared (FTIR) spectroscopy on a Perkin–Elmer System 2000 FTIR spectrometer. The studied films were deposited on both sides of Si(100) substrates (both sides polished). An uncoated substrate was used as reference.
Fig. 1. Growth rate of thin films of Co3O4 deposited at 186 °C as function of duration of pulse and purge times of Co(thd)2 (a, c) and O3 (b, d). Filled squares refers to Si (100) substrates and open triangles to soda–lime glass substrates. All measurements refer to the middle of the substrate (3.1 cm from first impact of the precursor on soda–lime glass and 2.3 cm on Si(100).
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Fig. 2. Growth rate as function of deposition temperature for Co3O4 films deposited during 1000 cycles on (a) soda–lime glass substrates and (b) Si(100). The film thicknesses in (a) at the points A, B, C and D, indicated in the inset, are measured by scanning, and XRR at point X is used for calibration of the scanner. The film thicknesses in (b) at the points F, M and X in the inset, are measured by XRR.
Fig. 4. Film thickness as function of number of deposition cycles on (a) soda– lime glass and (b) Si(100) when deposited at 186 °C. Thicknesses measured by XRF are given by stars. The film thicknesses on soda–lime glass are measured by scanning at the points A, B, C and D as indicated in the inset (a). Point X is measured by XRR for calibration of the scanner. The inset in (b) shows the measured points with XRR on Si(100) substrates.
Pelletized powder samples of Co3O4 and CoCO3 in a matrix of KBr were also measured by FTIR spectroscopy, using a blank pellet as reference.
keeping all others at values well within the ALD-growth regime (Fig. 1). The results show that a 1.5 s pulse of Co(thd)2 and a 6.0 s pulse of ozone is sufficient for ALD-type growth. The subsequent purging was kept at 1.0 s after a Co(thd)2 pulse and 1.5 s after an ozone pulse. It is apparent from Fig. 1 that even shorter purge times are sufficient to prevent CVD type growth. The growth rate was measured with XRR and scanning (Fig. 2). For both types of substrates, the growth rate varied only slightly with the temperature between 114 and 307 °C. The growth rate increased abruptly above 307 °C, indicating uncontrolled growth. The thermal decomposition temperature of the precursor was determined to be 310 ± 6 °C, according to the method described in Ref. [45]. Hence, the uncontrolled growth is likely to be a result of decomposition of Co(thd)2. The measured decomposition temperature is almost 100 °C lower than what was reported in Ref. [4]. The thicknesses of films deposited at 332 °C (Fig. 2) were estimated from optical scanning on soda–lime glass. Representative thickness gradients for depositions at 186 °C on soda–lime glass substrates are given in Fig. 3. It is apparent that the film thickness decreases with distance from the point of first impact between precursor and substrate. The decrease appears both along the horizontal and vertical flow directions. This probably reflects that the precursor is introduced to the reaction chamber from a point source rather than from a line source. The decrease in thickness within 2 cm along the flow direction from the leading end of the substrates is 2.6 and 9.3% for Si(100) and soda–lime glass, respectively. The Si(100)
3. Results The influence of pulse and purge parameters on the growth was investigated by changing one parameter at a time, and
Fig. 3. Thickness gradients for a Co3O4 film deposited at 186 °C during 1000 cycles along the length of the substrate, simulated using an optical scanned film. The thickness of the film is indicated in nm on the contour lines.
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Fig. 5. θ–2θ XRD data for films deposited on (a) soda–lime glass and (b) Si(100) as function of deposition temperature (138–283 °C).
substrates were measured by XRR, whilst soda–lime glass substrates were measured by optical scanning. Note that the uncertainty for the latter (∼ 10%) is much larger than for XRR (∼ 1%). The film thickness as function of deposition cycles was investigated in detail for a reaction temperature of 186 °C; pulse and purge times of 1.5 s pulse of Co(thd)2, 1.0 s purge with N2, 6.0 s ozone pulse, followed by a 1.5 s purge with N2. The film thicknesses were measured by XRR, optical scanning (calibrated with XRR), and XRF. The thickness and gradients at 200 cycles were measured by XRR, as these were below the detection limit of the optical scanner. The average thickness at 5000 cycles was measured by XRF, since this exceeds the upper limit for XRR. Fig. 4 reveals a linear relationship between the film thickness and the number of cycles for films deposited on soda–lime glass as well as on Si(100). The average growth rate on soda–lime glass is 0.022 ± 0.002 and 0.025 ± 0.003 nm/cycle as measured by XRR and XRF, respectively. For films deposited on Si(100) substrates, the corresponding rates are 0.020 ± 0.001 and 0.024 ± 0.002 nm/cycle, respectively. Representative XRD patterns are shown in Fig. 5. All films proved to be phase-pure Co3O4. However, the growth orientation was dependent on the deposition temperature and the type of substrate. For the lower part of the applicable 114 – 307 °C deposition range, the films show a tendency towards (100)-oriented growth on both types of substrates. At the high temperature end of this range, films
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Fig. 6. (a) FTIR spectra of films deposited in the range of 138–283 °C. Panel (b) shows spectra of powder samples of CoCO3 and Co3O4 for comparison.
deposited on Si(100) substrates show a (111)-oriented growth whereas films on soda-lime glass loose their preferred (100)orientation for depositions at 283 °C. A comparison of FTIR spectra of powder samples of Co3O4 and CoCO3 with that of Co3O4 films is given in Fig. 6. It is first noted that the FTIR spectra do not indicate presence of carbonate in the deposited films. Second, although the FTIR
Fig. 7. Roughness as function of number of deposition cycles on soda–lime glass (open squares) and Si(100) (filled squares) when deposited at 186 °C. Inset; Roughness as function of deposition temperature on soda–lime glass (open squares) and Si(100) (filled squares) when deposited with 1000 cycles.
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Fig. 8. AFM topography images for increasing number of deposition cycles for Co3O4 films on Si(100) and soda–lime glass when deposited at 186 °C.
spectrum for powder Co3O4 is very similar to those of the deposited films, the two main absorption peaks at 580 and 667 cm− 1 for Co3O4 powder are shifted slightly to 556 and 657 cm− 1 for the film. This shift towards lower wave numbers may be a result of the limited film thickness [46]. The variation of surface roughness as function of deposition cycles on soda–lime glass and Si(100), as measured by AFM, is shown in Fig. 7. For films deposited using 1000 cycles within the temperature range 114 – 283 °C, the average root mean square (RMS) roughness is 1.16 and 1.27 nm for depositions on soda–lime glass and Si(100), respectively, see inset in Fig. 7. The roughness is much larger for films deposited at 307 °C or above. The AFM topographies for some selected films are shown in Fig. 8. The size of the surface terminating crystals increases with the number of deposition cycles for both types of substrate. Their average diameter increases from 21 to 50 nm for film
deposited within 1000 to 40 000 cycles on soda–lime glass. Correspondingly, the average diameter increases from 22 to 62 nm for films deposited on Si(100) within 1000 to 40 000 cycles. The number of crystals at the terminating surface was obtained from AFM images as a function of deposition cycles, Fig. 9a. The results from the counting procedure have shown to be dependent on the quality of the AFM images, where a good quality will give a larger number of terminating crystals. The datasets in Fig. 9 are based on images with similar quality except for the measuring point at 10 000 cycles, which had a remarkable good quality. This point may thus be systematically larger than the others. The general trend, visualized by the fully drawn line, indicates a decrease in the number of crystals with increasing number of deposition cycles. The trend as drawn in Fig. 9a has a similar shape to that reported for simulations of comparable situations, Ref. [47]. The size of the crystals
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changes in the microstructure, as well as with the decomposition of the Co(thd)2 precursor at temperatures above 307 °C. The specific resitivity at room temperature for films deposited on soda–lime glass was evaluated as function of film thickness. Measurements were done at four positions along the flow direction, see inset in Fig. 11b. The thickness at the measuring positions was derived by optical scanning (calibrated by XRR and/or XRF). The thickness of the film deposited with 10 000 cycles was measured by XRF to be 241 ± 4 nm giving a specific resistivity of 5.9 ± 0.1 Ω cm, while that of 40 000 cycles was estimated (extrapolation of linear type growth) to be 940 ± 89 nm giving 80 ± 1 Ω cm. These results display a trend where the specific resistivity at room temperature versus film thickness goes through a minimum for a film thickness of some 40 nm before increasing with film thickness, which is surprising. 4. Discussion
Fig. 9. Number of surface terminating crystals for an area of 1 μm2 as function of (a) number of deposition cycles at 186 °C, and (b) function of temperature.
increase with increasing deposition temperature for a given number of cycles (Fig. 10), but the effect is less pronounced than what was found with respect to crystal size versus number of cycles, see Figs. 8 and 10. The number of crystals as a function of deposition temperature is given in Fig. 9b. Rocking curve analysis of films deposited on soda–lime glass was generally performed for the strongest reflection in each pattern shown in Fig. 5. The results are summarized in Table 1. These again emphasize that films deposited on soda– lime glass have a certain preferred (100)-orientation in the deposition range of 186–235 °C, whereas the film deposited at 138 °C shows a more random orientation. At 283 °C the preferred orientation seems to be lost. Films of Co3O4 deposited on Si(100) at 138 and 186 °C show the same tendency towards a preferred (100)-orientation. However, the preferred orientation is lost already at 235 °C. At 283 °C, the full width at half maximum (FWHM) decreases with ∼ 50% and the reflections become consistent with a (111)-oriented film. This is somewhat surprising as the surface of Si(100) is considered to be covered with an amorphous SiOx layer. However, φ-analysis of the film showed no in-plane orientation of the crystals. Specific resistivity at room temperature for films deposited on soda–lime glass is shown as function of deposition temperature in Fig. 11a. The specific resistivity increases from 0.14 to 1.12 Ω cm over the temperature range 114 – 259 °C, thereafter it increases significantly, reaching 3.27 Ω cm at 307 °C and 9.39 Ω cm at 332 °C. This increase correlates with
A possible mechanism for the thickness gradients as presented in Fig. 3 may be connected to the limited life time of ozone, which decomposes quite rapidly into O2 in the relevant temperature range. The half-life of ozone at 250 °C is only 0.03 s [48] and will be even shorter if collisions with other molecules are taken into account. This suggests that ozone will not diffuse uniformly within the whole reaction chamber. In addition, the film itself may catalyze decomposition of ozone, as is well known for MnO2 [48]. In this case, ozone is not likely to survive collisions with any surfaces in the reaction chamber. This mechanism is supported by the fact that we need several times the ozone flux to produce cobalt oxide films as what was necessary to produce calcium carbonate films with the same equipment [49]. The out-of-plane orientation for films deposited on Si(100) at 283 °C, and the lack of such orientation on soda-lime glass at the same deposition temperature, was surprising. The Si(100) substrates used in the current experiments are terminated by a layer of native amorphous SiOx. Hence, both types of substrates have an amorphous surface that should not impose oriented film growth. The difference in film orientation becomes apparent only at higher temperatures. This may indicate that the morphology of the crystals forming the film is altered from octahedrons to cubes as the temperature is increased [47]. This effect should, however, be substrate independent. The native amorphous SiOx layer on Si(100) is commonly in the range of 0.5–2 nm. It may thus be imagined that cobalt can diffuse through this layer and form arrangements that guide the film growth. However, we find it questionable that a Si(100) surface, which has fourfold symmetry, should direct the growth of the cobalt oxide into a (111) orientation, which has threefold symmetry. The FWHM data in Table 1 indicate that the tilt between the crystals is large regardless of temperature, with exception of the film deposited on Si(100) at 283 °C. Rocking curve analysis was similarly performed for the reflection with the highest intensity for the individual films deposited at 186 °C. The results are presented as function of deposition cycles (and film thickness) in Table 2. At these
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Fig. 10. AFM topography images of Co3O4 films on Si(100) and soda–lime glass substrates as function of increasing deposition temperature.
conditions (100)-oriented growth is achieved. The FWHM decrease slightly with increasing number of cycles on both types of substrates, with exception of the film deposited with 40 000 cycles. For the latter it was required to stop the deposition every 10 000 cycles to refill with precursor, which may be a cause for the increased FWHM. The preferred (100)-orientation at low temperatures can be explained if the crystallites take the morphology of octahedra. The surface of the films will then be terminated by {111} surfaces of Co3O4. The fastest growth direction would be along b100N which results in (100)-oriented films, as observed (cf. Ref. [47]). As the adopted AFM tips were spherical with a size comparable to that of the crystals (tip curvature radius of ∼ 20 nm), the proposed octahedron shape of the surface terminating crystals will be convoluted and appear as spheres in topography images. On the other hand, if the crystals really were shaped as spheres, a very low surface roughness should
have resulted (cf. Ref. [50]), which is not observed. It is thus likely that the surface terminating crystals have a specific shape, probably of an octahedron owing to the observed (100)preferred growth orientation at temperatures below 235 °C. If the surface terminating crystals kept the octahedron morphology for the (111)-oriented films, it would imply that the {111} surfaces of the octahedron were parallel with the substrate surface, leading to very low roughness. However, the observed roughness is significant, i.e. 1.94 nm (RMS). It is more consistent to assume that the surface terminating crystals grow as cubes, with their corners pointing outwards from the surface as in (111)oriented films, producing a notable surface roughness, see the inset in Fig. 7. An AFM image obtained for a film deposited at 259 °C on Si(100), shows triangular shaped crystals which are consistent with the corners of a cube oriented in the [111] direction, Fig. 12. The orientation of the triangular shaped crystals does not show any in-plane orientation which is consistent with
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Table 1 FWHM of XRD reflections for Co3O4 on substrates of soda–lime glass and Si(100)
Table 2 FWHM of XRD reflections for films of Co3O4 on substrates of soda–lime glass and Si(100)
Temperature (°C)
FWHM on soda–lime glass (°)
FWHM on Si(100) (°)
Number of cycles
138 186 235 283
14 (400) 12 (400) 14 (400) 18 (311)
14 (400) 13 (400) 13 (311) 7 (111)
FWHM on soda–lime glass (°)
FWHM on Si(100) (°)
3000 (∼70 nm) (311) 5000 (∼125 nm) (400) 10 000 (∼ 240 nm) (400) 40 000 (∼ 960 nm) (400)
22 19 17 21
19 18 15 21
Miller indices are given in parentheses.
Miller indices are given in parentheses.
the XRD φ-analysis. The lack of in-plane orientation also undermines a potential claim of a misshaped AFM tip. Growth of (111)-oriented Co3O4 from formation of cube shaped crystallites, has been reported earlier on substrates of Si (100) and/or glass [11,12,15,17]. In both cases the films were grown by MOCVD, using respectively Co(acac)3 (380–500 °C) and Co(acac)2 (375–550 °C) as precursors, notably at relatively high temperatures. Assuming that there is a conversion in growth regime from octahedron to cube morphology as the deposition temperature is increased, one would anticipate a minimum in surface roughness at the conversion temperature (see Ref. [47]). A small, yet detectable, minimum in surface roughness is observable for depositions on Si(100) around 250 °C, see inset in Fig. 7.
Fig. 11. Specific resistivity of Co3O4 films on soda–lime glass at room temperature as: (a) Function of deposition temperature for films deposited with 1000 cycles. (b) Function of film thickness for films deposited at 186 °C. Measuring points are indicated on the insets.
As evident from Fig. 8, the Co3O4 films deposited at 186 °C, with a preferred (100)-orientation, show an increasing size of the surface terminating crystals with increasing number of deposition cycles. This behaviour is typical for columnar growth. Amorphous surfaces, like soda–lime glass and in principle also Si(100), will initially orient the crystals randomly. Some of the crystals will have their highest growth rate in a direction normal to the substrate. These will in the end enclose the others completely (see Ref. [47]). As a result, the number of surface terminating crystals decreases, and the film surface will be dominated by crystals with their fastest growth direction normal to the substrate. As seen from Fig. 9b, the number of surface terminating crystals decreases with increasing temperature. The spread in tilt of the crystals also decreases slightly with increasing temperature. These observations are reasonable since the mobility of the adsorbed precursor fragments should increase with temperature, and hence also the tendency towards forming dense layers matching the crystallographic lattice of the underlying crystal. This will govern certain crystallographic surface terminations and more defined crystal habits will be developed. It might be intuitive that the film roughness should increase monotonically with increasing film thickness, however, as Fig. 7 shows, the film roughness does not follow this pattern.
Fig. 12. AFM topography image of Co3O4 deposited at 259 °C on a Si(100) substrate by 1000 cycles.
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The roughness increases towards a maximum for ca. 2500 cycles, before it goes through a local minimum and then appears to reach an equilibrium value. Care was taken during the measurements to prevent instrumental variations to have an impact on the results. The increase in specific resistivitiy as function of film thickness above some 40 nm is surprising (Fig. 11b). Normally, one would expect the resistivity to decrease with thickness due to better crystallinity. When comparing Figs. 7 and 11b it becomes evident that the observed minimum in film resistivity occurs in the same area where there is a maximum in surface roughness, indicating that the conduction is greatly affected by the microstructure of the film. According to the simulations in Ref. [47] the microstructure at this minimum should be of a fully covering film with a large proportion of grain-boundaries, which may indicate that the conduction paths are along the grain boundaries and not transverse the grains. The present results contradict those obtained by Kadam and Patil [26] for Co3O4 films deposited by spray pyrolysis. In addition to observing a decrease in specific resistance with increasing thickness, they found a much larger specific resistance (2.1 × 104 Ω cm for a 900 nm film) than for the present case (80 Ω cm, ∼ 940 nm). Similarly, a high specific resistivity of 316 Ω cm has been reported for 100 nm Co3O4 films deposited by spin coating [24]. The presently observed high resistivity for very thin films (200 cycles corresponding to 4 nm) is according to expectations since these consists of small crystallites with large relative thickness variations and may perhaps not be fully interconnected on the surface. 5. Conclusion Thin films of Co3O4 were made using Co(thd)2 (Hthd = 2,2,6,6-tetramethylheptan-3,5-dione) and ozone as precursors. Substrates of soda–lime glass and single crystals of Si(100) were used and ALD-type growth of Co3O4 was established in the temperature range 114–307 °C. All films are crystalline and with a preferred (100)-orientation at the low end of the temperature range, independent of the type of substrate. At the high end of the temperature range, films deposited on Si(100) showed (111)oriented growth, while films deposited on soda–lime glass substrates were unoriented. The orientation preferences are most likely accompanied by an octahedron-shaped morphology at low temperatures, and a cube-shaped morphology for films deposited on Si(100) at high temperatures. The measured specific resistivity on soda–lime glass substrates showed a non-monotonic dependence on film thickness with a minimum for films with a high proportion of grain boundaries, and surprisingly increased with increasing thickness. Acknowledgments This work has received financial support from the Department of Chemistry, University of Oslo, Norway. The authors are indebted to Oddvar Dyrlie for help and discussions concerning the AFM measurements, and to Cand. Scient. Martin Lie for XRF measurements.
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