Preparation of Al2O3 thin films by ion-beam-induced CVD: structural effects of the bombardment with accelerated ions

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Surface and Coatings Technology 80 (1996) 23-26

Preparation of A1203 thin films by ion-beam-induced CVD: structural effects of the bombardment with accelerated ions A. Caballero, D. Leinen, A. Fernhdez, A.R. Gonzhlez-Elipe* Instituto de CienciadeMaterialesdeSevilla(CSIC- Univ. Sevilla) andDept. Q.Inoygdnica,P.O.Box 1115,41080&villa, Spain

Abstract A&O, thin films werepreparedon fusedquartz by ion-beam-inducedCVD. Using this method, the substrateis bombardedwith O,+ ions (1 and 10keV) while a flow of Al(CH3)3 is directed onto its surface.The film! were very homogeneousand flat as observedby SEM. Examination by TEM showsa microstructureformed by small(d< 100.4), compactedgrains. Analysis of the local environmentaround aluminiumwascarried out for the films and for bombardedy-A1203samplesby X-ray absorption spectroscopy(EXAFS and XANES). The films were amorphous,but at 1273K they crystallized yielding the a-AlzOZ structure. High energy (10 keV) O,+ ions produce a lessheterogeneous environment around Al than low energy(1 keV) ions. The ability of EXAFS/XANES spectroscopyto characterizeion-beam-bombardedsubstratesis discussed. Keywords:

Thin films; X-ray absorption spectroscopy;Ion beamdeposition;Chemicalvapour deposition;Ion energy

1. Introduction Al,O, thin films are widely used as hard, protective coatings and have been prepared by several methods including ion-beam techniques [ 11. Using this method, an enhancement of the densification and homogenization of the films is produced [2]. Amorphization is another effect caused by bombardment with ion beams. Amorphous films can be investigated by X-ray absorption spectroscopy (XAS, XANES and EXAFS regions). This technique is sensitive to the local arrangement of atoms around a given atom. In this study, we have used XAS to investigate the local structure of Al,O, thin films prepared by ion-beam-induced CVD. This method involves the bombardment of a substrate with accelerated O2 + ions while a flow of an organometallic precursor is directed onto its surface [3]. The Al,O, films were characterized by SEM, TEM and XAS. The latter technique was also used to analyse the effect of ion bombardment on bulk y-A&O, used as a reference. For the films (amorphous according to XRD), the X-ray absorption spectra show differences in the coordination of Al as a function of the energy (1 or 10 keV) of the 02+ beam used for preparation, The formation of uA&O, occurs after annealing in air at TZ 1000 K, lower than the value usually reported for other A&O, systems [4]. * Corresponding author. Tel.: 34-5-4625626; fax: 34-5-4611962; e-mail: [email protected].

2. Experimental

details

A&O, thin films were prepared by ion-beam-induced

CVD [3] using Al(CH,), as a precursor. A hollow cathode gun (AGlO from VG) supplied with oxygen (P =2 x 10v5 Torr) and working at 1 or 10 keV was used for preparation. Under these conditions, the gun supplied a current of approximately 30 uA on a surface of about 2 cm’. Fused silica sheets were used as a support for the thin films. The characterization of the films was accomplished by SEM-TEM, XRD and XAS. The experimental conditions are given in Refs. [3] and [S]. The Al K-edge X-ray absorption spectra were taken at the SUPER-AC0 storage ring in LURE (Paris) by recording the drain current through the samples. Photon energies were selected with a quartz monochromator. This limited the photon energies available for the experiment to E < 1839 eV (energy of Si K-edge). The spectra were analysed by the usual methods, i.e. extraction of the EXAFS oscillations, calculation of the Fourier transforms (FTs) and fitting.

3. Results As observed by SEM, the A&O, thin films grown by ion-beam-induced CVD present a very flat, compact and homogeneous surface. The microstructure is shown in

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Fig. 1. It consists of a highly packed structure of very small grains (30-100 A) of hexagonal/circular shape. From XRD, the tilms are amorphous, although they yield the corundum structure after annealing in air at 1273 K. Fig. 2 shows the XRD diagram of a film annealed at this temperature. The peak pattern is practically identical to that of bulk CL-A&O,, thus showing the formation of this structure with no preferential crystallization in certain directions. The amorphization of solid targets subjected to bombardment with accelerated ions can be investigated by XAS. An example of this is shown in Fig. 3, where the XANES and FT curves of y-A&O, before and after bombardment with 02+ and Ar+ ions of 10 keV are presented. The XANES spectra of the bombarded samples were reproduced by a linear combination of the spectra of the original y-Al,O, (0) and amorphous Al,O, (A) (see Fig. 4). Combinations of 0.40 +0.6A and 0.50 + 0.5A yield the best reproduction of the experimental spectra of the samples bombarded with Arf and Ozf. It should be noted that the first peak of the spectrum, usually attributed to tetrahedrally coordinated aluminium (Al(IV)) [6,7], is not well reproduced because the spectrum of amorphous A&O, does not contain this feature. The peaks of the FT curves, in particular for the sample bombarded with Ari ions,

Fig. 1. Transmission electron micrograph prepared by ion- beam-induced CVD.

of an A&O,

thin film

a-Al,O,

Fig. 2. X-Ray diffraction diagram of an Al,O, thin tirn prepared by ion-beam-induced CVD after annealing at 1273 K.

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widen and their intensity decreases on ion bombardment. These features can be interpreted by assuming a large heterogeneity in the values of the Al-O distances and angles. Moreover, as shown in Table 1, the average A!-0 distances obtained by fitting decrease from 1.814 A to 1.796 A and 1.786 A for the OZt- and Ar’-bombarded samples respectively (owing to the very complicated coordination sphere around Al in y-A1203, these values must not be taken as accurate measurements, but as indicative of tendencies on bombardment). The local arrangement around Al was also analysed for the films prepared by ion-beam-induced CVD. Spectra were taken for films prepared using OZt ions of 1 and 10 keV and after annealing at 1273 K. Fig. 4 shows the XANES region of these spectra as well as those of y-A&O, and A1,TiOS samples used as references. The most significant observation from this series of spectra is that the first peak in the spectrum of y-A&O3 (i.e. 1566 eV), attributed to Al(IV) ions, does not depict such a high intensity in the rest of the samples. Only in the original films is there a small feature at this energy. However, its small height in comparison with the main peak suggests that, although some Al(IV) ions may exist in the films, most Al occupy octahedrally coordinated sites (Al(V1)). This is also supported by the similar position and relative intensity of the two main features of the spectra in comparison with those in the spectra of cc-A&O, [ 6,7] and AIPTiOS where only Al( VI) exists. However, the peaks in the film and A12Ti05 spectra are wider than in the spectrum of a-AlzOa. In A12TiOS, the local structure around Al is a distorted octahedron with three different Al-O distances (see Table 1). Therefore the similar shape of the spectra of the two films and Al,TiO, suggests that, in the films, there is also a large heterogeneity of Al-O bond lengths. The spectrum of the film prepared with O2 + ions of 1 keV was used to reproduce the spectra of the bombarded y-A&O3 samples in Fig. 3. To study further the local environment around Al in the films, we analysed the EXAFS region of the X-ray absorption spectra. Fig. 4 shows the FTs obtained from the EXAFS oscillations. In comparison with the FT curve of a-AIZOs, the other FTs are wider. The same happens with the FT of y-A&O3 in Fig. 3. The smallest intensity of the first peak of the FTs is found in yAl,O,-Arf and in the film prepared with 1 keV ions. In y-A1203, the first peak of the FT is wide because in the first coordination sphere around aluminium there are several aluminium sites (i.e. Al(V1) and Al(IV)) and Al-O and Al-Al distances (see Table 1). However, the large FT peaks of the two ion-beam-induced CVD films, where according to XANES most sites have octahedral coordination, must be the result of amorphization of the atomic network. As shown for bombarded y-A&O,, this generally produces an increase in the width of the FT peaks and a decrease in their intensity. The low intensity

A. Caballero et al./Surface and Coatings Technology 80 (1996) 23-26

I

,

1s 1550

8

19 1560

9

13 1570

9

I 1580

T

3

1 1590

t

8

I 1600

Energy I eV

(4

Fig. 3. XANES (left) and FT curves (right) of y-A&O3 before and after bombardment with Ar’ and 0, + ions of 10 keV. Broken curves: sum spectra of y-Al,O, + amorphous A&O,.

F

Al K

r-Al,O, 10 keV

------ /II \

AI,TiO,

I I I I t I! 1550 (4

1560

1570

1

I I I I I

1580

1590

I I I I I,, 1600

Energy I eV

0

(b)

I r , , , , , (

2

4

6

RIA

Fig. 4. XANES (left) and FT curves (right) of A&O3 thin films prepared with Ozc ions of 1 and 10 keV and after annealing at 1273 K. XANES spectrum of A1,TiOS is included for comparison, The spectrum of the annealed flm is similar to that of cr-A&O:,.

of the peaks corresponding to the second coordination sphere, especially in y-Al,03 and in the film prepared with 1 keV ions, is characteristic of the amorphous state, indicating the loss of order at this distance from the central atom. To obtain additional insight into the amorphization phenomenon, analysis was performed by fitting the first peak of the FTs. Unfortunately, the short range in K available in the EXAFS spectra means that

reliable information can only be obtained on the relative values of the bond distances. Table 1 summarizes the values obtained, together with the crystallographic Al-O distances in the reference compounds. From these values, it appears that in the bombarded samples the average Al-O distances are always shorter than in the original materials (i.e. y-A&O, and a-A&O,). Moreover, the smallest distances are found for samples where, accord-

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Table 1 Average AI-O distances obtained by fitting of the first peak of the FT curves. Crystallographic distances of w&O,, y-Al,O, and Al,TiO, are included for comparison

y-A&0,/0: y-A&OS/Art cr-Al,O, Film, 10 keV Film, 1 keV Al,TiO,

d” (A)

db (A)

1.975 (Al(vr)), 1.710 (Al@)) -

1.814

1.972, 1.852 2.092, 1.921, 1.821

1.796 1.786 1.878 1.929, 1750’ 1.765 -

a Crystallographic distances. b Distances obtained by fitting. ’ Best fitting obtained with two Al-O distances (average value, 1.809 A). This is consistent with the large width of the first peak of the FT curve.

ing to the intensity and width of the FT peaks, the heterogeneity in the coordination of Al is higher.

4. Discussion and conclusions The preparation of oxide thin films by ion-beaminduced CVD requires the decomposition of an organometallic precursor by accelerated 02’ beams. CO2 and H,O residues formed by decomposition of the precursor are removed, while a metal oxide tilm progressively grows during bombardment [ 31. For the preparation of Al,03 thin films, we used Al(CH,), as precursor. It is expected that the films prepared in this way will be homogeneous and dense, as usually observed when a film is bombarded with a beam of accelerated ions [2]. The analysis by TEM of the microstructure of the films confirms that the Al,O, films are compact and homogeneous. The microstructure is a compact agglomeration of small grains of 30-100 A. Although no crystalline order exists in these grains, a corundum structure is directly obtained on annealing in air at relatively low temperature (1273 K). The formation of c+AIZOZ in fibres and other similar systems starts at 1273-1373 K and usually requires temperatures as high as 1473 K to achieve complete crystallization [4]. XANES and EXAFS analysis of the films has revealed some clues about their amorphous state. An important point is that XANES shows that the coordination

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around Al is octahedral. A large heterogeneity of local sites is another feature that can be deduced from the shape of the XANES spectra and from the intensity and width of the FT curves. According to a visual inspection of these curves, the amorphization of the films is higher when using ions of 1 keV. Ar’ is more effective than Ozf in producing this effect, as revealed by analysis of bombarded y-A&O, samples. From the fitting analysis, we have found that a large heterogeneity in the bond distances is accompanied by a decrease in the average Al-O bond length. Another conclusion is that a less heterogeneous arrangement of atoms is formed in ionbeam-induced CVD films prepared with OZf ions of 10 keV. We have found a similar effect for TiOz ionbeam-induced CVD films where a mixture of rutile and amorphous structures is obtained with ions of this energy [ 51. The reason why high energy ions lead to an enhancement in the crystallization of the films is not fully understood. The high energy ion cascades generated by primary ions must penetrate deeper into the growing flms. Thermalization of this energy at the end of ion tracks may provide the means to enhance the crystallization of the films during the growth process.

Acknowledgements We thank CICYT (project no. MAT94-1039-C02-01) for financial support. The XAS measurements at LURE were possible due to an HCM grant from the EU. The transmission electron micrograph was taken at the “Servicio de Microscopia de la Universidad de Sevilla”.

References [l] [2] [ 31 [S] [5] [6] [7]

Y. Takahashi, K. Nitobe, J. Uramoto, T. Momose and H. Ishimaru, J. Voc. Sci. Technol. A, I1 (1993) 1491. P.J. Martin, J. Mater. Sci., 21 (1986) 1. D. Leinen, J.P. Espinos, A. Fernbndez and A.R. Gonzalez-Elipe, J. Vat. Sci. Technol. A, 12 (1994) 2728. P.G. Lucuta, J.D. Halliday and B. Christian, J. Mater. Sci., 27 (1992) 6053. A. Caballero, D. Leinen, A. Fernandez, A. Justo, J.P. Espinos and A.R. Gonzalez-Elipe, J. Appl. Ph~‘s., 77 (1995) 591. Ph. Ildefonse, R.J. Kirkpatrick, B. Montez, G. Calas, A.M. Flank and P. Lagarde, Clays Clay Miner., 42 (1994) 276. K.J. Roberts, J. Robinson, T.W. Davies and R.M. Hooper, Jpn. J. Appl. Phys., 32 (1993) 652.