Growth of Fe on Si (100) at room temperature and formation of iron silicide

lh'n ELSEVIER

Thin Solid Films 280 (1996) 171-177

Growth of Fe on Si (100) at room temperature and formation of iron silicide K. Rtihrnschopf, D. Borgmann, G. Wedler * Institute of Physical and Theoretical Chemistry, University of Erlangen-Nuremberg, D-91058 Erlangen, Germany Received 20 June 1995; accepted 15 November 1995

Abstract Low-energy electron diffraction (LEED), Auger electron spectroscopy and X-ray photoelectron spectroscopy (XPS) investigations of both the growth of an iron film on silicon (100) at room temperature and the subsequent formation of iron silicide are the subjects of this paper. An in-situ cleaned silicon (100) wafer without carbon or oxygen contamination exhibiting the known 2 × 1 reconstruction in the LEED pattern served as the substrate. Iron was deposited on this reconstructed surface at 300 K. The comparison of theoretical calculations based on three growth mechanisms with XPS data obtained with take-off angles of 0° and 50° clearly demonstrates a layer-by-layergrowth of the iron film on silicon (100). At 300 K no formation of iron silicide was observed, although an interaction between iron and silicon could be detected at the interface. The formation of iron silicide was observed at annealing temperatures of 630-730 K. Quantitative XPS analysis yields the presence of FeSi2, when the thickness is large enough. Neither the iron film on silicon nor the iron silicide shows any LEED pattern. Keywords: Growth mechanism; Iron; Silicides; X-ray photoelectron spectroscopy (XPS)

1. Introduction The extraordinary importance of silicon to technical applications, especially in microelectronics, has led to a huge number of publications dealing with its chemical, structural and electronic properties. The interaction of metals with the silicon surface is of high interest regarding the formation of metal/silicon contacts as well as the chemical reaction under formation of silicides. Among the silicides, special attention has been paid to FeSi2, because there are two stable phases, the metallic o~and the semiconducting/3 phase with a bandgap of about 0.85 eV [ 1,2 ]. Formation of iron silicide was achieved by solid-phase epitaxy (SPE) on Si (100) [3-5] and on Si ( 111 ) [ 1,6-8], by molecular beam epitaxy (MBE) [9], by chemical vapor deposition (CVD) [ 10] and by ion beam synthesis [ 11,12]. Different FeSi2 phases were fo:.':ned usually by choosing special annealing conditions. Von K~inel et al. also report on a transition between different phases on silicon (111) [ 13]. Reflection high-energy electron diffraction (RHEED) investigations show an epitaxial growth of/3-FeSi2 on silicon (111) [14,15]. The aim of this work is the investigation of the growth mechanism of iron on silicon (100) at 300 K and the reactiv* Corresponding author. 0040-6090/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10040-6090 ( 95 ) 08248-4

ity of this system at room temperature and at higher temperatures. These studies have been performed with iron overlayers of various thicknesses. Quantitative X-ray photoelectron .~pectroscopy (XPS) analysis was used to determine the stoichiometry and homogeneity of the formed iron silicide liims.

2. Experimental The experiments were carried out in a VG-ESCALAB 200 analysis system providing a base pressure of 7 × 10- ~~mbar. The UHV chamber was also equipped with a deposition system consisting of an iron source and a thickness rate meter in order to control the evaporation rate, a surface analysis system with facilities for XPS, Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS) and lowenergy electron diffraction (LEED), and with a sputter gun for cleaning procedures. Furthermore there are two heating systems, one for cleaning the sample at high temperatures ( 1 200-1 700 K) and the other one for heating procedures at lower temperatures (up to 1 000 K). XP spectra were obtained using Mg Ka radiation (20 mA, 15 kV) when the growth of the iron film on silicon was studied and AI Ka radiation (27 mA, 15 kV) when the formation of iron silicide was followed. The XPS spectra were measured with take-off

172

K. Riihrnschopf et al. /Thin Solid Films 280 (1996) 171-177

angles of 0* and 50 ° with respect to the surface normal. The hemispherical electron analyzer was operated at a constant pass energy of 20 eV. In the AES experiments the electron energy of the primary electrons was 3 kV. The sample, a Si (100) wafer with dimensions of 10 × 10 × 0.5 mm 3, was mounted in a specially designed sample holder with cooling tank and a tungsten coil as heater, which allowed cooling to 85 K and indirect heating to 1 000 K with a constant rate of 3 K s- ~ during the experiments. The silicon samples in the as-received state carry the native SiOz layer and a carbon contamination on the top of the surface. Therefore the sample was at first cleaned by Ar ÷ bombardment at 300 K until the carbon was completely removed and only a small amount of SiO2 remained on the silicon surface. This thin SiO2 layer was removed by an RTA process (rapid thermal annealing by additional electron beam heating) at 1 500--1 600 K followed by fast cooling to lower temperatures. Iron was deposited on the cleaned reconstructed Si (100) 2 × 1 surface at 300 K from a pure iron coil with constant deposition rates ( 1-3 nm h - ~with different experiments) by means of direct electrical heating. The evaporation source contains an aperture system, which allows an equal deposition of iron onto both the silicon substrate and a quartz crystal of a rate meter to control the iron deposition rate. The iron silicide was formed by several annealing steps. Depending on the thickness, temperatures of 630 K for thin films or 730 K for thick films were necessary to form a homogeneous iron silicide layer.

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3. Results and discussion

Fig. !. XP spectra showing the growth of an iron film on silicon (100) at 300 K; (a) Fe 2p, (b) Si 2p.

3.1. Growth of iron films at 300 K

for the determination of the peak areas, the energetic peak position, the full width at half maximum (FWHM) and the subtraction of the background, whereas the subtraction of the XP satellites was carried out with standard VG software. The intensity of a photoemission line i of an element k is given by Eq. (1) [ 16]"

Both the growth of iron on silicon and the tbrmation of iron silicide were studied by means of the Fe 2p and Si 2p photoemission signals. Fig. 1 shows the increase of the iron peaks (a) and the decrease of the silicon signal (b) at several iron deposition steps at 300 K. The total coverage of iron in monolayers (ML) after repeated deposition procedures is shown in Fig. 1 at the right side of the two spectra. In this paper a coverage of 1 ML is equivalent to the number of iron atoms in the (110) plane of a bee iron crystal. This plane shows the highest density of 1.7 × 10 Is atoms cm-2. The signals observed in the case of 28 ML iron proved to exhibit the same line shape, energetic position and intensity as those of 40 ML or thicker deposited films. The Si 2p signal could not be observed in the XP spectra at these iron coverages. During the whole deposition experiments neither SiO2 (Si 2p, 103 eV) nor iron oxide (Fe 2p3/2, 710.8 eV) could be detected. Iron is detectable down to about 1/50 ML and the XP peaks can be quantified down to about 1/ 10 ML. The XP spectra were quantified after subtraction of a linear background and the appropriate XP satellites. An available IBM-compatible computer program (origin 3.0) was used

o0

'ik~Fo'ikPil¢Tlkf Nkex~''.AiJ¢

--Zcos ~ ) d z

(1)

z=O

I~.k is the measured intensity, F is a function containing the mean X-ray flux, the detector efficiency, which is constant for a constant pass energy, the analyzed sample area and the acceptance angle of the analyzer, o',.k is the photoionisation cross-section [ 17 ]. P~,kis the asymmetry term defined by Eq. (2): 1 P~.k= 1 --~fl~,k(3COS2 '0-- 1)

(2)

where/3o, is the asymmetry factor [ 18] and 0 the angle between X-ray source and the path of the photoelectrons. The transmission function T~.kof the spectrometer depends on the kinetic energy (Ek,0 of the photoelectrons and was tested

!73

K. Riihrnschopf et al. / Thin Solid Films 280 (1996) 171-177

before the measurements to be -"-r,-- ~~'-0.65 a, • Nk is the concentration of the element k (depending on the depth z), tp is the angle between the surface normal and pt, otoelectron path. Ai.k, the inelastic mean free path (IMFP), was calculated according to Tanuma et al. [ 19]. The intensity of the photoemission of an overlayer A (Fe) with a layer thickness d on a sample B (Si) is obtained by integration of Eq. ( 1) from 0 to d: I(A) i,I,

=

Fori,~Pi,kTi,~kA(A)i.k Xcos ~ ( 1 - e x P ( A (A)Tdcos tp))

(3)

The iron overlayer thickness was calculated from the relationship between the measured intensity I(A)~.k and the intensity l(R)~.k of a reference sample (cleaned polycrystalline piece of iron) with identical experimental parameters (Eq. (4)). The measured intensity of the photoemission of a thick iron film (more than 7 nm) proved to be equivalent to that of the pure polycrystalline iron material in the case of the two applied take-off angles within the experimental error. Furthermore the intensities meast~red at take-off angles of 0 ° and 50 °, respectively, of the Fe 2P3/2 and the Fe 3p signals show the same ratio for thick iron films and for the reference material. Therefore the use of a polycrystalline iron material is legal as a reference in this case, because a dependence of the measured intensities on diffraction effects associated to escaping electrons cannot be found [ 20,21 ].

3

<4)

Fig. 2 shows the exponential increase in the intensities of the Fe 2p3/2 peak and the decrease in the intensities of the Si 2p peak for the take-off angles q; = 0 ° and 50 °. The intensities have been normalized to the highest intensity measured with the same experiment. The take-off angle could be varied by means of a turn of the sample. This procedure influences the l

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described function F, which, however, is without any consequence on account of the chosen evaluation method. The number of monolayers was obtained from the thickness by means of the known crystallographic data [22]. XPS data are more sensitive to the surface than to the bulk in case of large take-off angles (50°). Therefore measurements with a bilayer system (iron film on bulk silicon) lead to both a faster increase in the emission from the overlayer and a faster decrease of the emission from the substrate for a take-off angle of 50 ° than for 0 °, when the film grows and no interdiffusion takes place. In the other case, when simultaneous with the deposition, due to very fast interdiffusion, a homogeneous compound is formed throughout the total XPS detectable depth, the ratio of the intensitie~ of both elements in the compound are independent of the take-off angles. The influence of the take-off angle following ~'.i'om the curves, which are shown in Fig. 2, proves the existence of a bilayer system without such an interdiffusion at 300 K. The experimental data are compared with theoretical curves calculated for three different growth mechanisms (Fig. 3). The first model describes a layer-by-layer growth, ,~,hereas the other models are based on the one hand on a vertical growth of islands which cover 50% of the silicon surface and on the other hand on the formation of 50% of a complete monolayer of iron, which has been deposited on the

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0

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Fig. 3. Intensityof the Fe 2p3/2(a) and of the Si 2p signal (b) in dependence on iron coverage (full line) as comparedwith theoretical valuescalculated for three growth mechanisms (interrupted lines). Take-offangle of the photoelectrons to the surface normal,0°.

174

K. Riihrnschopf et al. /Thin Solid Films 280 (1996) 171- 177

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silicon surface before island growth starts, respectively. Calculations c. " : intensities were carried out for the increase of the Fe 2p3/2 peak with Eq. (5) and for the decrease of the Si 2p peak with Eq. (6). The procedure to obtain reference data in the case of iron has been described above. Reference data for the silicon have been obtained from both a cleaned 2 × 1 reconstructed Si (100) substrate and an Ar+-bom barded Si (100) surface. Measurements of these two references provide the same intensities within the experimental error for the applied take-off angles.

A(A) ~.kcos I(B)j.,= exp(

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The measured curves for iron and siliconshow a very good agreement with the theoretical one for the layer-by-layer growth. In contrastto [3 ] interdiffusionof the two elements could not be observed. Also no formation of iron silicide takes place at 300 K as follows from the factthatthe intensity of the Si 2p signal approaches zero when the layer thickness is large enough. Nevertheless, quantitativeXPS data of the Fe 2p3/2 peak in Fig. l indicate an interactionbetween iron and siliconfor low ironcoverages. Fig. 4 describesthe change as well of the FWHM as of the shiftof the binding energy of the Fe 2p3/2 peak in dependence on the coverage of iron. For small coverages, especially up to one monolayer, the values of the FWHM are lower by about 0.65 eV and those of the binding energy ~ higher about 0.4 eV than those of iron coverages exceeding 9 ML. A FWHM of 2.2 eV and a binding energy of 706,8 eV in case of the Fe 2p3/2 peak observed with thicker films conespond exactly with the data of an iron crystal. The iron layers on the top contribute more to the whole signal at a take-off angle of 50* than for 0*. Consequently, when a take-off angle of 50 ° is used the interaction between iron and silicon at the interface can no longer be detected, if 6 or more monolayers have been deposited. With

a take-off angle of 0 ° this effect is obtained after deposition of 9 ML. Only in the range up to 6 and 9 monolayers, respectively, the measured Fe 2P3/2 signal is a superposition of the emission from pure ir~= and from iron which interacts with silicon. This interaction cannot be observed with the Si 2p signal since this signal is too strongly dominated by the bulk signal. LEED investigations do not indicate an epitaxial growth of iron on silicon (100) with a long-range order at 300 K up to a coverage of about 50 ML. The 2 × 1 superstructure could be seen up to half a monolayer of iron. At the coverage of one monolayer, only a 1 × 1 structure was observed, whereas in case of three or even more monolayers no LEED pattern could be observed. Lagomarsino et al. [ 23 ] studied the structure of evaporated iron films on silicon (111). They did not observe any LEED pattern as long as the film thickness did not exceed some nanometers of iron. However, a 1 x I iron bulk structure could be observed with iron layers of about 30 nm thickness. The absence of a LEED pattern in the case of small thicknesses is explained by the existence of domains, which are separated by translation vectors periodic with the substrate but not with the overlayer lattice.

3.2. Formation of iron silicide Iron silicide was formed by deposition of iron on a clean and reconstructed Si (100) surface at 300 K and subsequent annealing to various temperatures mainly in two experiments. In one experiment the iron silicide formation was started after deposition of a thin film ( 2 ML), in the other one with a thick iron film (40 ML). Fig. 5 refers to the thin iron film. Three annealing steps in the range of 630 to 830 K were applied for a period of 20-25 rain in each case. The formation of the silieide is already recognizable after the first anne~.ling step (630 K, 20 min) by a change in the peak intensities. The intensity of the iron signal decreases, while that of the Si 2p peak (not shown in Fig. 5) reaches a value lying between the intensities of pure

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Fig. $. XP spectra of the Fe 2p core level taken at various temperatures during transformation of a thin ~ron film into a silicide.

K. Riihrnschopf et aL / Thin Solid Films 280 (1996) 171-177 ia)

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duringtransformationof a thickiron filmintoa silicide, silicon and silicon with the iron overlayer. The most important information is provided by the form of the Fe 2p spectra (Fig. 5). Iron silicide shows a plasmon loss, which has also been described by different authors (see, for example, Ref. [24] ), a shift to higher binding energy ( +0.2 eV) and a smaller FWHM ( - 0 . 3 eV). Annealing at higher temperatures (730, 830 K) does not cause any detectable differences in the spectra. It was not possible to observe a LEED pattern of the iron silicide. Likewise no LEED pattern was observed in the second experiment, in which the formation of a thicker i r e silicide film was studied starting with 40 monolayers iron on silicon (Fig. 6). Silicon cannot be detected with XPS, when such a thick iron overlayer is present. Contrary to the behavior of the thin iron film, annealing to 630 K for 45 and 105 min does not lead to silicide formation within the depth, which can be detected with XPS. Indeed there is a little change in both the FWHM and binding energy of the Fe 2p3;2 peak at 630 K for 105 rain, but the changes are not significant enough to prove the formation of iron silicide. There is no change in the inter~;ities of iron and silicon and the plasmon loss in the Fe 2p spectra does not appear. However at annealing at 730 K for 60 rain the reaction between iron and silicon has been completed. The silicide formation is recognizable by the appearance of the plasmon loss in the Fe 2p spectra and the change of the peak intensities of both elements. The intensity of the iron peak decreases and the silicon peak can be

175

observed again. Further annealing for 60 min at the same temperature does not change anything in the spectra. The final formation of the silicide can be detected clearly. These changes indicate that silicon diffuses into the iron layer, not vice versa. In the case of an iron diffusion into the silicon the intensity of the iron signals should decrease with increasing annealing time at the formation temperature because of the diffusion gradient into the silicon hulk. This behavior should be observable particularly for thin iron films (e.g. 2 ML), when a thick, stabilized phase of an iron silicide could not be formed. The experiments concerning the formation of both thick and thin iron silicide films show that there is no further decrease of the peak intensities of iron after the completion of the silicide formation, even when the sample is annealed for long times. As compared with pure silicon the Si 2p peak of iron silicide has shifted to a higher binding energy ( +0.2 eV) and the FWHM has become wider ( +0.15 eV). A shift of the Fe 2p3/2 peak to higher binding energy ( + 0.4 eV) is obtained as well as a decrease of the FWHM ( - 0.55 eV). Quantitative evaluation of the XPS data indicates the presence of a homogeneous phase. In the case of a random distribution of both elements the integration of Eq. ( 1) yields Eq. (7) provided that the thickness of the film is much larger than the attenuation length of the photoelectrons. li.k = Fo'i.:,Pi.~Ti.~NkAi.~ cos tp

(7)

The ratio of the measured intensities of two photoemission lines ( i , j ) of two elements (k, l) in a homogeneous sample, in this case of the Si 2p and Fe 2P3/2 lines, is given by Eq. (8):

1, k ¢YLAP,.kTi.kNkA,.k -=_-

lj,,

(8)

The product of the factors o-: P, T and A is usually called the sensitivity factor S. The ratio of the element concentrations in the sample can consequently be calculated by Eq. (9): N~=/~.__.~,l

(9)

In the present case the calculation yields the composition FeSi~_ cf the iron silicide layer. Fig. 7 shows the change of the FWHM and the shift of the Fe 2pa/2 peak in dependence on various annealing steps during the formation of thin and thick iron silicide films. It is obvious, that the temperature of the silicide formation depends on the iron oveflayer thickness. The peak shift ( +0.4 eV) and the decrease of the FWHM ( -0.55 eV) of the Fe 2p3/2 peak of FeSi, as compared with the values of a thick iron film is in very good agreement with that of a monolayer iron deposited at 300 K. Therefore iron at low coverages on silicon should have a similar interaction with the silicon as iron in FeSi,. This is important especially for gas adsorption. The described changes are smaller in case of the thin iron silicide film (peak shift: +0.2 eV; FWHM:

K. Riihrnschopf et al. / Thin Solid Films 280 (1996) 171-177

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iron causing a higher electron density in the outer shells and therefore a higher intensity of the L3M45M45peak of iron.

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%_. Annealing Steps Fig. 7. FWHM and peak position of the Fe 2p3/2 peak in dependence on the annealing steps. Thin film: (I) 300 K; (II) 630 K, 20 rain; (III) 730 K, 20 min; (IV) 830 K, 25 min. Thick film: (I) 300 K; (2) 630 K, 45 min; (3) 630 K, 105 min; (4) 730 K, 60 min; (5) 730 K, 120 min.

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Fe on Si (300 K) Iron sEcide (730 K; 120 rain)

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200

400

600

800

1000

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film on Si (100) intoFeSi2at 730 K. - 0.3 eV). This is easy to understand, because at a coverage of 2 monolayers the iron layer has an initial interaction with the substrate. The formation of iron silicide can also be deduced from the AE spectra (Fig. 8). Starting with a 40 ML iron film deposited at 300 K the formation of FcS~2 follows as well from the appearance of the SiLMMpeak, which cannot be seen after deposition of 40 ML iron, as from a variation of the FeLMM peak, The intensity of the iron signals decreases and of the plasmon losses increases. Additional to this, there is a change in 1he shape of the peaks. For a thick iron film, as for an iron crystal, the L3M~.3M45peak is the strongest one. In contrast to that in the AE spectrum of iron silicide the L3M4sM4s peak shows the highest intensity, Changes in the intensities of the FeLMM peaks after iron silicide formation have also been observed by other authors [ 25,261. The effect sliown in Fig. 8 has also been reported by Viefhaus [ 27 ] who explains the changes by an electron transfer from silicon to

Iron which is deposited onto a clean 2 × 1 reconstructed silicon (100) surface, grows in a layer-by-layer mechanism at 300 K. This is the result of the evaluation of XPS data of iron and silicon as compared with three calculated growth mechanisms. Neither an interdiffusion of both elements nor a formation of iron silicide could be detected at this deposition process. There is an interaction between silicon and iron in the case of thin iron films of about 1 or 2 ML, especially for submonolayer coverages, which follows from a change of the FWHM and a shift of the binding energy of the Fe 2p3/2 core level. The XPS data of the Fe 2p3/2 core level are comparable with the values of FWHM and binding energy of the iron signal after formation of an iron silicide. The Si 2p peak does not change concerning the FWHM and binding energy, because the biggest part of the signal intensity is due to the photoelectrons of the bulk silicon. The iron silicide is formed after annealing the sample at various temperatures with changing annealing times. The complete formation up to the surface depends on both the thickness of the iron overlayer and the annealing temperature. Starting with 40 ML iron the silicide was formed after annealing at 730 K for 60 min, whereas a 2 ML iron film on silicon was transformed into iron silicide after annealing at 630 K for 20 min. The evaluation of XPS data measured with takeoff angles of 0 ° and 50 ° points to a homogeneous iron silicide layer with the stoichiometry FeSi:. LEED investigations do not show an epitaxial growth in a long-range order, so that diffraction spots could not be observed, neither for the iron overlayer on silicon at room temperature nor for the formed iron silicide.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (SFB 292).

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