The growth and characterization of iron silicides on Si(100)

Surface Science 251/252 (1991) 59-63 North-Holland

59

The growth and characterization J.M. Gallego,

J. Alvarez,

J.J. Hinarejos,

of iron silicides on Si( 100)

E.G. Michel and R. Miranda

Departamento de Fisica de la Materia Condensada C-III, Universidad Authoma

de Madrid E-28049 Madrid, Spain

Received 1 October 1990; accepted for publication 16 December 1990

We have studied and characterized different iron silicides grown in situ on a Si(lOO)-2 x 1 surface. Silicide films have been produced both by solid-phase epitaxy, and by reactive deposition epitaxy, keeping the substrate at several hundreds of degrees during the growth. The grown silicide layer was characterized by different surface sensitive techniques. Two different types of s&ides (FeSi and FeSi,) were identified by directly determining the atomic stoicbiometry of the compound, as well as by studying the evolution of the characteristic plasmon loss structure of the compounds. Finally, ISS has revealed the existence of a Si-enriched layer on top of the sihcide grown by solid-phase epitaxy.

1. Introduction

obtain FeSi, are much lower in the case of RDE than in the case of SPE.

Part of the interest that metallic silicides have attracted in the past years has recently moved to semiconducting ones, due to their possible applications in opto- and micro-electronic devices.

Among the semiconducting silicides [1,2], &FeSi, seems to be a promising”candidate [3,4]: its narrow and direct band gap of 0.87 eV [5] makes it adequate for the development of near-infrared detectors and emitting devices. Besides, the close match of its unit cell with that of silicon could facilitate the growth of epitaxial films of &FeSi, on Si(100) wafers and thus allow the fabrication of these devices within the silicon microelectronics technology [6]. We have grown in situ iron silicides on Si(100) by means of solid-phase epitaxy (SPE) and reactive deposition epitaxy (RDE). We have characterized two types of silicides (FeSi and FeSi,), which grow depending on the preparation conditions. The s&ides and their properties were studied with different surface-sensitive techniques, and by comparing them with the corresponding bulk single crystals. The temperatures needed to ~39-6028/91/$03.50

2. Experimental The experiments were carried out in two ultrahigh-vacuum (UHV) chambers. The first one was equipped with a cylindrical mirror analyzer for Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS), a low-energy electron diffraction (LEED) optics, a Kelvin probe and a quadrupole residual gas analyzer. In the second one, X-ray photoemission spectroscopy ultraviolet photoemission spectroscopy (XPS), (UPS) and ion scattering spectroscopy (ISS) experiments could be performed. The samples used were Si wafers (p-type, 10” cme3) oriented the [lOO] direction. They were cleaned by cycles of Ar+ sputtering and annealing to 900°C. After a few cycles a sharp two-domain 2 x 1 LEED pattern was obtained, with no traces of contamination within AES and XPS sensitivity. Fe was evaporated from a wire wrapped around another resistively heated W wire. The XPS spectra were

0 1991 - Elsevier Science Publishers B.V. (North-Holland)

J.M. Gal/ego Ed al. / Characterizaiion

60

taken by using the Mg Ka line (photon 1253.6 eV).

energy =

of iron srlicides on Si(100) is the spectrometer take-off angle (0 o ). In a similar way the Fe signal can be fitted to the equation I,, = Z”(1 - exp( -d/h,,

3. Growth of Fe on Si(100) at room temperature The mode of growth and the coverage calibration were determined by AES and XPS. In fig. 1 the evolution of the intensities of the Fe2p and Si2p XPS peaks (the latter in logarithmic scale) is depicted versus the Fe evaporation time. The Si signal follows an exponential decay according to the expression Is, = Za exp( -d/‘hsi

COS Q),

(1)

as shown by the linear dependence on logarithmic scale, which corresponds to a layer-by-layer type of growth (assuming a constant sticking coefficient). I, is the substrate signal without attenuation, d is the thickness of the iron film, h,, is the inelastic mean free path (IMFP) of the Si photoelectrons escaping through the Fe overlayer, and +

cos (p)),

(2)

where I” is the signal of a bulk Fe sample. To obtain I” we deposited a Fe layer thick enough to attenuate the Si2p signal from the substrate below the XPS sensitivity. We obtained finally Xsi (kinetic energy = 1154 eV) = lt.5 A and h,, (kinetic energy = 540 eV) = 7.2 A. The exactness of these values and the coverage calibration provided by them was cross-checked in the following ways: (1) comparison with the expected theoretical values from different calculations [7]: (2) comparison with the expected XPS signal for one ML of Fe (atomic density 1.2 x 1015 atoms/cm*); (3) comparison with the experimentally measured signals for known coverages of other adsorbates previously studied with our experimental setup [8.9]. Furthermore, a closer inspection reveals the presence of breaks in the Fe2p signal, which can be assigned to the completion of the subsequent layers of Fe (see lower panel of fig. 1). As the surface sensitivity for an electron energy over 1000 eV is rather low, the sharpness of the subsequent breaks (additional to the first one) is limited. Thus, the straight segments in the lower pannel of fig. 1 have been traced in agreement with the previous calibration. This procedure is justified by the fact that these breaks have been more clearly observed with AES [lo]. In conclusion, our results indicate that Fe grows layer by layer on Si(100) at room temperature. Concerning the quality of the growth, ISS experiments [ll] revealed the presence of Si at the surface after the deposition of several monolayers of Fe. The AES results [lo] showed a Si signal stronger than expected for an ideal layerby-layer growth. which makes it reasonable to assume that some Si is dissolved in the growing Fe film.

4. Formation of iron disilicide 0

100 EL%=ORATION

150 TIME

200 (5)

250

Fig. 1. Intensities of Fe2p (lower part) and Si?p (upper part) XPS peaks as a function of the Fe evaporation time onto a Si(100) substrate at room temperature.

Several methods have been reported for growing silicides: solid-phase epitaxy (deposition of the metal film on the Si substrate and subsequent annealing), reactive deposition epitaxy (deposition

J.M. Gallego et al. / Characterization of iron silicides on Si(lO0)

of the metal on a substrate heated to a determined temperature), coevaporation of the metal and silicon and subsequent annealing, coevaporation on a hot substrate, etc. We have studied the formation of iron s&ides and the identification of the phases formed in the two first cases.

61

Fe/Si (100) 4.0 z cn

3.0 -

o

25 “C

.

235 ‘C

0

350 “C

0

0 0

4. I. Solid-phase

\

epitaxy

In a previous publication [ll] we identified the formation of FeSi after annealing several Fe monolayers deposited at RT to temperatures in the range of 425-55O’C. Further annealing between 625 and 775 “C gave rise to the formation of FeSi, [ll,lO]. In fig. 2 the evolution during the annealing process of the FeLW Auger peak at 702.6 eV is shown. The appearance of a conspicuous plasmon loss can be clearly observed. The evolution of this plasmon loss and its energy shift during the annealing are displayed in the inset of fig. 2. In the temperature region corresponding to the forma-

Fe LVV

0.0

675

665

695

705

715

KINETIC ENERGY (eV) Fig. 2. FeLW Auger peak for a Fe thick layer deposited at room temperature (bottom) and annealed to 770 ’ C (top). The evolution of the plasmon loss during the annealing process is shown in the inset. From bottom to top the annealing temperature is: RT, 370, 435, 500, 560, 620, 690 and 770 o C, respectively.

f 0

Fig. 3. Evolution of the ratio between the Fe,, and Si,, AES peaks during the deposition of Fe onto a Si(100) substrate maintained at different temperatures.

tion of FeSi the energy distance to the the main peak is 21.9 eV, while in the temperature region corresponding to FeSi, a value of 20.7 eV is obtained. We measured the values of the plasmon losses in the corresponding single crystals. For FeSi(lOO) we obtained 22.0 eV and for FeSi,(lOO) 20.9 eV, in nice agreement with our previous assignment. The values reported [12] for the plasmon losses of bulk samples of FeSi and FeSi, are 22.0 and 21.2, respectively. This further confirms our previous statements on the formation of FeSi at 450 o C and FeSi 2 at 650 o C for this film. 4.2. Reactive

665

4.0 2.0 COVERAGE (ML)

deposition

epitaxy

Fig. 3 shows the evolution of the ratio between the intensities of the Fe,, and Si,, AES peaks when iron is deposited on the Si substrate maintained at several temperatures. When the sample is kept at room temperature, the value of this ratio indefinitely grows until the Si signals disappears, which is expected for a Fe film of increasingly larger thickness. On the contrary, when the samples are kept at 235 and 350 o C during the deposition, the Fe/Si ratio reaches a value that remains constant for further deposition (0.24 and 0.54, respectively), after increasing during the first

1. hf. Galiego et al. / Characterization

Fe&i(lOO)

1OOeV

EP-

T-350°C

*

cleanSi +O.tiML +I.2 ML +1.&IML +2.5 ML +3.7 ML 4.9 ML +6.8 ML +8.0 ML +9.2ML

1

I

10 Fig. 4. EELS spectra

1

30 20 ENERGY LOSS (eV)

during the deposition maintained

I!

of Fe on Si(lO0)

at 350 o C.

stages. This can be interpreted assuming that a strong intermixing of Fe and Si atoms takes place during the growth, and reflects the formation of compounds of well-defined stoichiometry. The identification of the possible phases formed is difficult and cannot be directly deduced from the value of the ratio, as this value is not constant through all the grown film. Depth profiling experiments (not shown) have indicated that for samples grown at 350°C the value of the Fe/Si ratio reaches a value of - 0.4 inside the sample, revealing the presence of a Si-enriched layer close to the surface. The value of 0.4 approximately corresponds to a bulk stoichiometric sample of FeSi, (the theoretical values of these ratios are 0.87 for a bulk sample of FeSi and 0.44 for a bulk sample of FeSiz). The formation of a well-defined compound is confirmed by EELS data. Fig. 4 shows the evolution of the loss spectra when iron is deposited on a

of iron silicides on Si(iOO)

Si substrate maintained at 350 o C. The first spectrum corresponds to the clean Si sample, and shows the known features of a bulk plasmon loss near 17 eV, the surface plasmon loss at 11 eV, an interband transition near 5 eV and three surfacestate transitions near 2, 8 and 1.5 eV, characterisdc of the clean ordered surface [13]. After the deposition of 0.6 ML of Fe the surface-state transitions disappear and the plasmon losses are greatly reduced. Subsequent deposition causes the disappearance of all the characteristic losses of the Si substrate, and the progressive growth of a new set of peaks at 20.4, 13.9, 6 and 3.3 eV. Althou~ the resolution of our EEL spectra is - 0.5 eV, the value at 20.4 eV is close to that measured by us for a bulk sample of FeSi, (20.9 eV). So we ascribe the losses at 20.4 and 13.9 to the bulk and surface plasmon losses of FeSi,. In conclusion, EELS and AES results indicate that P-FeSi, can be formed evaporating Fe onto a Si substrate maintained at 350 o C. The EEL spectra for the sample prepared at 235’ C display a similar evolution compared to those shown here. However. the depth profiling experiments show a different behaviour. In this case the maximum of the Fe/Si ratio is located at the surface, and progressively diminishes towards the interior of the sample. As the value of the Fe/Si ratio corresponds in this case to a film of FeSi, we consider that in this case no Si-enriched layer is formed. So, the only difference between the samples grown at 350 o C and at 235 o C is the presence in the last case of a Si-enriched layer, which may facilitate a further growth of Si on the FeSi 2 film.

Acknowledgements Financial support by the ESPRIT BRA 3026 and the CICYT is gratefully acknowledged. We thank Professor Derrien for fruitful discussions and communication of results prior to publication. References [l] A.E. White, K.T. Short and D.J. Eaglesham, Lett. 56 (1990) 1665.

Appl.

Phys.

JM.

Gallego et al. / Characterization of iron silicides on Si(lO0)

[2] R.G. Long and J.E. Mahan, Appl. Phys. Lett. 56 (1990) 1655. [3] J.E. Mahan, K.M. Geib, G.Y. Robinson, K.G. Long, Y. Xinghua, G. Bai, M.-A. Nicolet and M. Nathan, Appl. Phys. Lett. 56 (1990) 2126. [4] N. Cherief, C. D’Anterroches, R.C. Cinti, T.A. Nguyen Tan and J. Denien, Appl. Phys. Lett. 55 (1989) 1671. [S] M.C. Bost and J.E. Mahan, J. Appl. Phys. 58 (1985) 2696. [6] N. Cherief, R. Cinti, M. de Crescenzi, J. Derrien, T.A. Nguyen Tan and J.Y. Veuillen, Appl. Surf. Sci. 41/42 (1989) 241. [7] David R. Penn, J. Electron Spectrosc. Relat. Phenom. 9 (1976) 29.

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[8] E.G. Michel, M.C. Asensio and S. Ferrer, Surf. Sci. 55 (1988) L385. [9] E.G. Michel. J.E. Ortega, E.M. Oellig, M.C. Asensio, J. Ferron and R. Miranda, Phys. Rev. B 38 (1988) 13399. [lo] J.M. Gallego and R. Miranda, J. Appl. Phys. 69 (1991) 1377. [ll] J. Alvarez, J.J. Hinarejos, E.G. Michel, J.M. Gallego, A.L. Wzquez de Parga, J. de la Figuera, C. Ocal and R. Miranda, Appl. Phys. Lett.. submitted. [12] B. Egert and G. Panzner, Phys. Rev. B 29 (1984) 2091. [13] J.E. Rowe and H. Ibach, Phys. Rev. Lett. 31 (1973) 102.