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Langmuir 2002, 18, 1206-1210
Nanostructure of Vapor-Deposited
57
Fe Thin Films
A. Ve´rtes,*,†,‡ Gy. Vanko´,‡ Z. Ne´meth,† Z. Klencsa´r,‡ E. Kuzmann,‡ Z. Homonnay,† F. H. Ka´rma´n,§ E. Szo¨cs,§ and E. Ka´lma´n§ Department of Nuclear Chemistry, Eo¨ tvo¨ s Lora´ nd University, Budapest, Hungary, Research Group of Nuclear Methods in Structural Chemistry at the Eo¨ tvo¨ s Lora´ nd University, Hungarian Academy of Sciences, Budapest, Hungary, and Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary Received July 3, 2001. In Final Form: November 18, 2001 The structure and oxidation process of iron thin films vapor-deposited on different substrates were investigated by means of 57Fe conversion electron Mo¨ssbauer spectroscopy (CEMS). It was found that the direction of the emerging magnetic anisotropy displayed by the iron film is influenced by the surface treatment applied to the substrate material in advance of the deposition process. Magnetically ordered interphase layers were detected between the substrates and the evaporated iron layers. In the interphase layers, iron had an oxidation level of 3+. On its surface, the vapor-deposited iron had an amorphous oxide or/and hydroxide layer whose corrosion resistance was tested by exposure to water as well as to atmospheric oxygen.
Introduction Thin iron layers can display different structural and magnetic properties depending on the method of preparation, as well as on the physical parameters and experimental conditions applied in the actual preparation process.1,2 For the present study, thin layers of enriched 57 Fe were prepared by vapor deposition on the surface of various substrates. In the experiments Al and Cu metals, as well as Cu/Zn, Al/Si, and Sn/Pb alloys and SiO2/Si wafers, were used as substrate materials. The samples were investigated by the method of 57Fe conversion electron Mo¨ssbauer spectroscopy (CEMS) and atomic force microscopy (AFM). Experimental Section Two different types of surface treatment were applied to the substrates in advance of the evaporation. One set of samples was cleaned by aqueous alkaline solution containing 40% NaOH for 2 min and rinsed in deionized distilled water. Another set of samples was mechanically polished with silicon carbide papers down to 2400 grid, followed by washing in ethanol (Table 1). Iron enriched in 57Fe (90%) was vapor-deposited on substrates using Hochvakuum-Bedampfungsanlage B 30.2 (Dresden) evaporation equipment at a pressure of 2 × 10-3 Pa. (Natural iron contains 2.7% of 57Fe, which is the Mo¨ssbauer-active iron isotope.) The temperature of the substrate was Ts ≈ 300 K at the beginning of the deposition process. The temperature was not controlled during the deposition. 57Fe conversion electron Mo ¨ ssbauer spectroscopy measurements were carried out on the deposited samples by using a 57Co(Rh) source with 109 Bq activity. Both the source and the samples being investigated were kept at room temperature during the measurements. The conversion electrons were counted by a Ranger SD-300 electron detector filled with the appropriate mixture of He and CH4. The 57Fe Mo¨ssbauer isomer shifts are reported relative to R-iron. the standard deviations of the fitting * Author to whom correspondence should be addressed. † Eo ¨ tvo¨s Lora´nd University. ‡ Research Group of Nuclear Methods in Structural Chemistry at the Eo¨tvo¨s Lora´nd University, Hungarian Academy of Sciences. § Chemical Research Center, Hungarian Academy of Sciences. (1) Johnson, M. T.; Jungblut, R.; Kelly, P. J.; den Broeder, F. J. A. J. Magn. Magn. Mater. 1995, 148, 118-124. (2) van Diepen, A. M.; den Broeder, F. J. A. J. Appl. Phys. 1977, 48 (7), 3165.
parameters are given in parentheses as the normal error of the least significant digits. Parameters quoted without standard deviations were fixed during the fitting process. The Mo¨ssbauer spectra were analyzed by the MossWinn program.3 AFM investigations were performed on a Nanoscope III AFM (Digital Instruments) in contact mode in air. A commercially available cantilever with a force constant of 0.58 N m-1 and a Si3N4 tip was used during the measurements.
Results and Discussion The effects of the treatment of the substrate surface are shown in Figure 1. The AFM images clearly demonstrate that the etched (by NaOH) surface is more ragged than the mechanically polished one. The roughness profiles of the surfaces can be characterized by the RMS (root mean square) of the Z values (vertical position, as shown in Figure 1) within the given area and they are calculated by
RMS )
∑(Zi - Zavg)2
x
N
where Zavg is the average of the Z values, Zi is the current Z value, and N is the number of points within the given area. The calculated values are 111 and 325 nm for the mechanically polished and NaOH-treated surfaces, respectively. On the basis of the surface area (∼200 cm2) covered by the evaporated 57Fe and the weighed amount of 57Fe used for the evaporation, the thickness of the layers was calculated to be 200 ( 50 nm. In a single evaporation run, seven to nine deposited samples were prepared. For one of the runs, the thickness of the evaporated iron layer was also measured by section analysis of the AFM images taken from an iron layer vapor-deposited on a SiO2/Si wafer with very low roughness. The obtained line profile (Figure 2) gives quantitative information on the thickness of the iron layer, which is approximately 170-200 nm, in accordance with the calculations. Some of the measured Mo¨ssbauer spectra of the evaporated samples are shown in Figure 3. The corre(3) Klencsa´r, Z.; Kuzmann, E.; Ve´rtes, A. J. Radioanal. Nucl. Chem. 1996, 210 (1), 105.
10.1021/la011057h CCC: $22.00 © 2002 American Chemical Society Published on Web 01/18/2002
Nanostructure of Vapor-Deposited
57Fe
Thin Films
Langmuir, Vol. 18, No. 4, 2002 1207
Table 1. Mo1 ssbauer Parameters of the Evaporated Samplesa
sample 1
substrate (composition) Al (99.9%) Al (99.9%)
2 3
Sn/Pb (30/70%)
4
SiO2/wafer
5
SiO2/wafer
6
Cu/Zn (65/35%)
7
Cu (99.9%)
8
carbon steel
9
Al/Si (88/12%)
compound
area of spectrum components (%)
isomer shift δ (mm s-1)
quadrupole splitting ∆ (mm s-1)
magnetic field H (T)
line width Γ (mm s-1)
R-iron interphase surface oxide R-iron surface oxide R-iron interphase surface oxide R-iron interphase surface oxide R-iron interphase surface oxide R-iron interphase surface oxide R-iron interphase surface oxide R-iron interphase surface oxide R-iron interphase surface oxide
69.5 18.4 12.1 93.5 6.5 69.4 9.9 20.7 77.2 2.0 20.8 71.9 4.1 24.0 65.4 13.0 21.5 67.6 9.4 23.0 74.4 2.0 23.6 56.2 14.9 28.9
0.00(1) 0.4 0.30(1) -0.0120(7) 0.13(2) -0.005(1) 0.4 0.36(1) -0.0024(4) 0.4 0.37(1) -0.002(1) 0.4 0.33(2) -0.006(2) 0.4 0.32(2) -0.005(1) 0.4 0.33(2) -0.0051(9) 0.4 0.37(2) -0.0075(7) 0.4 0.349(4)
1.56(2) 1.43(3) 1.24(2) 1.23(3) 1.32(4) 1.27(3) 1.27(3) 1.22(6) 1.131(8)
32.8(1) 36.0(1) 33.703(6) 32.846(8) 35.2(5) 32.919(5) 36.20(7) 32.883(9) 36.2(2) 32.85(1) 34.0(5) 32.84(1) 34(1) 32.945(7) 35.93(9) 32.776(5) 34.0(2) -
0.328(1) 1.37 1.35(3) 0.513(2) 0.90(4) 0.416(3) 1.37 1.49(7) 0.424(2) 0.23(4) 1.94(8) 0.427(4) 0.5(1) 1.8(1) 0.417(5) 1.37 1.31(6) 0.419(5) 1.37 1.53(10) 0.420(3) 0.19(4) 2.2(1) 0.418(2) 1.37 1.38(2)
intensities of the 2nd and 5th lines relative to the 3rd and 4th lines
Θb
1.2
43.4
3.1
69.3
3.7
80.0
4
90.0
4
90.0
3.9
83.7
4.0
90.0
3.6
77.0
3.56
75.9
a The thickness of the evaporated 57Fe was 200 ( 50 nm. Sample 1 was cleaned by aqueous alkaline solution containing 40% NaOH for 2 min and rinsed in deionized distilled water. All other components were mechanically polished with silicon carbide papers down to 2400 grid and washed with ethanol b Angle enclosed by the directions of the magnetic field and of the advance of the γ-photons.
Figure 1. AFM profile of pretreated substrates before evaporation. Horizontal positions are measured in the plane of the substrate. (a) Mechanically polished Al surface, RMS ) 110.83 nm. (b) Mechanically polished and etched (by NaOH) Al surface, RMS ) 324.62 nm.
sponding Mo¨ssbauer parameters are collected in Table 1. The decomposition of the spectra resulted in three components: two with magnetic structure and one with a quadrupole doublet representing a paramagnetic component. To obtain the reproducibility of the measurements, parallel studies were carried out on SiO2/Si wafer samples 4 and 5. The obtained results are reassuringly similar (see Table 1). The most striking differences can be seen
among the areas of the spectrum components and among the angles enclosed by the directions of the hyperfine magnetic field and of the advance of the γ-photons. The six absorption lines of a magnetically split 57Fe Mo¨ssbauer spectrum have intensities depending on the angle Θ between the direction of the γ-rays and the hyperfine field at the 57Fe nucleus, i.e., the local magnetization. The intensity ratios (from left to right) are I1:
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Langmuir, Vol. 18, No. 4, 2002
Figure 2. Boundary curve of the 57Fe layer deposited on a SiO2/Si wafer as derived on the basis of AFM measurements. The 0-nm layer thickness corresponds to the substrate surface. Horizontal positions are measured is measured in the plane of the substrate.
I2:I3:I4:I5:I6 ) 3(1 + cos2 Θ):(4 sin2 Θ):(1 + cos2 Θ):(1 + cos2 Θ):(4 sin2 Θ):3(1 + cos2 Θ). In our experiment, the γ-rays were always perpendicular to the macroscopic plane of the sample. Consequently, an intensity ratio of 3:0:1:1: 0:3 implies magnetization perpendicular to the sample, whereas a ratio of 3:4:1:1:4:3 means that the magnetization lies in the plane of the evaporated layer. In polycrystalline samples, in the absence of texture effects, the I1:I2:I3 ratio is given by
∫0π3(1 + cos2 Θ) sin Θ dΘ:∫0π4 sin2 Θ sin Θ dΘ:∫0π (1 + cos2 Θ) sin Θ dΘ ) 3:2:1. The main magnetic component of the spectra, displayed in Figure 3, represents R-iron. The ratio I1:I2:I3 differs from that expected for a polycrystalline material, indicating that the samples have a preferred magnetic orientation. Magnetic Orientation. It is well-known that the axis of easy magnetization lies predominantly in the plane of the thin iron layers. It was demonstrated experimentally1,2 that factors such as the roughness of the substrate surface; the formation of interface alloys; and the patchiness, grain size, and texture of the thin layers can affect the magnetic orientation. Our measurements suggest, as demonstrated by the data in Table 1, that treatment by aqueous alkaline solution (40% NaOH for 2 min) on the surface of the Al substrate before the evaporation of iron promotes the formation of the perpendicular magnetic anisotropy. When a mechanical polishing of the surface of the substrate was carried out, the magnetic orientation became parallel to the plane of the iron layer in most cases. The appearance of perpendicular magnetic anisotropy can be explained as follows. The alkaline treatment causes roughness on the surface of Al. (See Figure 1b.) The first part of the evaporated iron results in the formation of an interphase (see the next paragraph) that, together with the metallic iron layer, is expected to follow the grooves of the rough substrate surface. On one side, this results in stress in the iron layer and the magnetostriction forces the magnetization direction out of the plane of the sample.4 On the other side, because of the unevenness of the surface, (4) Taka´cs, L.; Ve´rtes, A.; Leidheiser, H. Phys. Status Solidi A 1982, 74, K45.
Ve´ rtes et al.
the angle between the direction of the γ-rays and the normal of the surface can differ from 0°. As a consequence of both phenomena, in the Mo¨ssbauer spectra, the area ratio of the absorption lines deviates from the 3:4:1:1:4:3 value that would be expected if all magnetic moments were aligned perfectly to the plane of the substrate material. Interphase Formation. An interesting and important result of this work is that interphase layers were detected between the substrates and the evaporated layers. Iron in these interphase layers has an isomer shift of ∼0.4 mm s-1 that represents the Fe3+ oxidation state. The corresponding local magnetic field and the line width are 35(1) T and 1.3(2) mm s-1, respectively. These parameters suggest that the surface oxide layers, combined with the adsorbed water originally on the surface of the substrates, and the first portion of the evaporated iron together form an iron oxyhydroxide interphase. It is considered that this layer is a precursor of R-FeOOH, that is, the bulk goethite has about a 39-T magnetic field.5 The lower magnetic field (35 T) might be due to the small magnetic particles and the high concentration of crystal defects, which can decrease significantly the effective magnetic field experienced by 57Fe.6-8 The area ratio of this spectrum component is in the range of 0-18%, as shown in Table 1. Taking into account the facts that this layer is sandwiched between the substrate and the evaporated iron layer (so that the conversion electrons formed in these layers have to cross the entire thickness of the evaporated iron layer to be detected) and that the Debye-Waller factor of the amorphous-like R-FeOOH is less than that of R-iron by ∼30%,9 the portion of evaporated iron that forms the interphase can be estimated. Liljequist used Monte Carlo simulation to derive a correlation between the detected intensity of the conversion electrons and the depth of their origin.10 To calculate the interphase thickness, we used the results of Liljequist’s publications11 and the DebyeWaller factors of 57Fe for various iron compounds. As the Debye-Waller factors need to be estimated as well,9 only a very rough estimation can be made for the relative occurrence of the phases in vapor-deposited iron layers. On the basis of the corresponding relative subspectrum areas in the measured conversion electron Mo¨ssbauer spectrum for the present case, the portion of evaporated iron that forms the interphase was estimated to be between 0 and 40%. Heat treatment (at 200 °C in air) did not change the parameters of this spectrum component to any significant extent (see later in the text). This result supports the interpretation that this component represents an interphase layer that is not accessible by atmospheric oxygen. Surface Oxidation. The third component in the CEMS spectrum of the vapor-deposited samples is a quadrupole doublet with an isomer shift of 0.30-0.35 mm s-1, quadrupole splitting in the range of 1.2-1.5 mm s-1, and a line width in the range of 0.90-2.2 mm s-1. The relatively large line width suggests that this component is a (5) Cohen, R. L., Ed. Applications of Mo¨ ssbauer Spectroscopy; Academic Press: New York, 1976; p 92. (6) Morup, S. J. J. Appl. Phys. 1976, 11, 63. (7) Morup, S.; Dumesic, J. A.; Topsoe, H. Magnetic Microcrystals. In Applications of Mo¨ ssbauer Spectroscopy; Cohen, R. L., Ed.; Academic Press: New York, 1980; Vol. II. (8) Gerward, L.; Morup, S.; Topsoe, H. J. Appl. Phys. 1976, 47, 822. (9) Meisel, W.; Kreysa, G. Z. Anorg. Allg. Chem. 1973, 395, 31. (10) Liljequist, D. J. Phys. D 1983, 16, 1567. Liljequist, D.; Ismail, M. Phys. Rev. B 1985, 31, 4131. Liljequist, D.; Saneyoshi, K.; Debusmann, K.; Keune, W.; Brand, R. A.; Kianka, W.; Ismail, M. Phys. Rev. B 1985, 31, 4137. (11) Ve´rtes, A.; Kuzmann, E.; Ve´rtes, Gy.; Szo˜kefalvy-Nagy, Z.; Ka´tay, E.; Mezey, G. Hyperfine Interact. 1988, 42, 1013.
Nanostructure of Vapor-Deposited
Figure 3.
57Fe
57Fe
Thin Films
Langmuir, Vol. 18, No. 4, 2002 1209
CEMS spectra of vapor-deposited iron layers: (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 9.
Figure 4. 57Fe CEMS spectra of sample 1 kept at room temperature in water for (a) 30 min and (b) 180 min.
Figure 5. 57Fe CEMS spectra of sample 1 kept at T ) 200 °C in atmospheric oxygen for (a) 30 min and (b) 60 min.
superposition of two or more doublets representing different chemical compositions. The large value of the quadrupole splitting is indicative of a very asymmetrical environment formed around iron atoms as a result of the high concentration of defects and the inhomogeneous stoichiometry. Consequently, the corresponding thin layer can be identified as consisting of amorphous iron oxide or/and hydroxide. The thicknesses of these layers were similar (Table 1), which supports the idea that their formation took place on the outer surface of the evaporated iron. An estimation of the thickness values based on calculations published in refs 10 and 12 shows that a 1020-nm-thick iron layer was transformed to surface oxide.
Treatment of the samples in distilled water for times ranging from 30 min to 3 h did not increase the thickness of this layer, as demonstrated in Figure 4. This shows that the spontaneously formed amorphous oxide layer on the surface of the evaporated iron has a relatively high corrosion resistance. Figure 5a and b displays the 57Fe CEMS spectra of the iron film exposed to oxygen at 200 °C for 30 and 60 min, respectively. According to Figure 5a, 30 min of exposure to oxygen led to the appearance of Fe3O4 at the expense of R-iron. The magnetic field values (Table 2) of 42 and 46 T observed for the octahedral and tetrahedral sites, respectively, of the Fe3O4 component are smaller than the expected ideal values of 46 and 49.3 T.5 This is an indication of superparamagnetic particles and of a high concentration of lattice defects. Because, at a given
(12) Salvat, F.; Mayol, R.; Martinez, J. D.; Parrellada, J. Nucl. Instr. Methods B 1985, 6, 547.
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Ve´ rtes et al.
Table 2. Effect of Exposure to Oxygen at T ) 200 °C on the Composition of Sample 1, as Seen by 57Fe CEMSa component sample R-iron
sample 1, as prepared I ) 69.5(2) % δ ) 0.00(1) mm/s H ) 32.8(1) T I ) 18.4(2) % δ ) 0.4 mm/s H ) 36.0(1) T -
sample 1 kept for 30 min in oxygen
I ) 46.8(2) % δ ) 0.00(1) mm/s H ) 33.0(1) T interphase I ) 16.9(3) % δ ) 0.4 mm/s H ) 33.8(1) T Fe3O4 I ) 20.9(3) % δT ) 0.27 mm/s HT ) 45.4(1) T ∆T ) 0.01 mm/s δO ) 0.67 mm/s HO ) 42.0(1) T ∆O ) 0.04 mm/s surface oxide I ) 12.1(2) % I ) 15.4(1) % δ ) 0.30(1) mm/s δ ) 0.34(1) mm/s ∆ ) 1.56(2) mm/s ∆ ) 1.26(2) mm/s
sample 1 kept for 60 min in oxygen I ) 43.9(2) % δ ) 0.00(1) mm/s H ) 33.0(1) T I ) 16.6(2) % δ ) 0.4 mm/s H ) 34.0(1) T I ) 23.4(3) % δT ) 0.27 mm/s HT ) 46.0(1) T ∆T ) 0.01 mm/s δO ) 0.67 mm/s HO ) 42.6(1) T ∆O ) 0.04 mm/s I ) 16.1(1) % δ ) 0.36(1) mm/s ∆ ) 1.20(2) mm/s
a I, δ, ∆, and H denote the relative area fraction of the corresponding spectrum component, the 57Fe isomer shift, the quadrupole splitting, and the hyperfine magnetic field, respectively. The subscripts T and O refer to the tetrahedral and octahedral coordinated sites of magnetite, respectively
temperature, a correlation exists between the size of the superparamagnetic Fe3O4 particles and the hyperfine magnetic field observed in a Mo¨ssbauer experiment,13 the size of the magnetite particles can be estimated on the basis of the observed magnetic field values. For the present case, the particle size of Fe3O4 was estimated to be around 10 nm. As reflected by Figure 5b, a further 30 min of exposure did not alter the composition considerably. This shows that, during the first 30 min of exposure, the majority of the surface of the iron film is covered with iron oxides, which prevent further oxidation. (13) Szabo´, D.; Czako´-Nagy, I.; Ve´rtes, A. J. Colloid Interface Sci. 2000, 221, 166.
During exposure to oxygen, magnetite was formed almost exclusively at the expense of R-iron. That is, the amount of the magnetic component with a magnetic field of 34-36 T was not changed considerably. This finding is in accordance with the assumption that the latter magnetic component originates from an interphase layer that was formed at the beginning of the deposition process. The quadrupole splitting of the surface oxide decreased 11 the heat treatment in oxygen (Table 2), and this can be explained by the formation of a more uniform chemical structure. For example, the 2Fe(OH)3 f Fe2O3 + 3H2O transformation can take place. Conclusions Vapor-deposited thin iron layers were found to have a complex structure. For the preparation conditions described in this paper, it has an interphase between the substrate and the metallic layer. The interphase displays a hyperfine magnetic field of ∼35 T, and it is considered to be R-FeOOH. There is a magnetic orientation in the iron. The preferred magnetic orientation in the vapor-deposited thin iron layers can be either perpendicular or parallel to the plane of the sample depending on the pretreatment of the surface of the substrate. The formation of an amorphous iron oxide and/or hydroxide takes place on the surface of the deposeited iron. Exposure to oxygen at elevated temperature results in the formation of nanosized magnetite. Acknowledgment. This work was supported by the Hungarian Science Foundation OTKA (T 034983 and T 034839) and by the Hungarian Academy of Sciences (AKP 2001-121). LA011057H
