Chemical beam epitaxy of iron disilicide on silicon

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C R Y S T A L G R O W T H

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Journal of Crystal Growth 146 (1995) 444-448

Chemical beam epitaxy of iron disilicide on silicon J . Y . N a t o l i , I. B e r b e z i e r , A . R o n d a , J. D e r r i e n * Centre de Recherche sur les M&anismes de la Croissance Cristalline 1, CRMC2 - CNRS, Campus de Luminy, Case 913, F-13288 MarseiUe Cedex 9, France

Abstract

The growth of high quality semiconducting/3-FeSi 2 layers on silicon substrates is rather difficult due to a large lattice mismatch (up to ~ 5.5% on Si(111)) and a very different crystallographic structure (an orthorhombic structure on top of the diamond one). We report on a new method using the chemical beam epitaxy (CBE) technique to stabilize at first the tetragonal a-FeSi 2 phase (lattice mismatch ~ 0.8% on Si(lll)) at ~ 550°C. Then a post-annealing up to ~ 650°C induces a phase transition from the a- to /3-phase via a tremendous coalescence of numerous small metallic a-grains (~ 200 A in width) into large semiconducting /3-grains (< 1 ~m in width) of high quality, suitable for Si integrated optoelectronic technology.

1. Introduction

Due to its optical gap close to 0.85 eV ( ~ 1.46 p,m) the semiconducting/3-FeSi 2 phase may be a potential candidate for optoelectronic applications directly integrated in a Si chip. This was the reason why during the last few years its heteroepitaxy on Si substrates was a very active field of research [1,2]. During the course of this heteroepitaxy, due to the strain field exerted by the Si substrates on ultra-thin epilayers and also the probable influence of the local atomic chemical bonding at the interface, various pseudormorphic a n d / o r strained FeSi 2 phases have been stabilized at low thickness [2-6]. With increasing thickness a n d / o r increasing temperature these metastable phases usually relax towards the sta-

* Corresponding author. i Laboratoire associ6 aux Universit6s Aix-MarseiUe II et III.

ble/3-FeSi 2 phase in equilibrium on top of the Si substrate in the low temperature range ( < 940°C) of the bulk F e - S i binary phase diagram [4]. Although various growth techniques, including the solid phase epitaxy (SPE) [1,2,5,6], reactive deposition epitaxy (RDE) [4], molecular beam epitaxy (MBE) [3], chemical beam epitaxy (CBE) [2,7,8], ion implantation and annealing [9,10], alloepitaxy [11], atmospheric chemical vapour deposition [12,13], etc., have been used in order to achieve high quality/3-FeSi2 epilayers, the results are still to be improved. So far/3-FeSi 2 epitaxial grains obtained directly by those techniques are either too thin and too narrow in lateral dimensions for optoelectronic applications, or mostly composed with various azimuthal epitaxial orientations, displaying a high concentration of defects and a rough/3-FeSi2/Si interface. We report on a new technique to achieve large /3-FeSi 2 grains (about one micrometer of lateral dimensions), well oriented on top of a S i ( l l l ) surface by preparing with CBE a metastable ho-

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J.Y. Natoli et al. /Journal of Crystal Growth 146 (1995) 444-448

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2. Experimental procedure

mogeneous film of small a-FeSi 2 grains (about ~ 200 A in width). By slowly post-annealing this a-film at a higher temperature ( ~ up to 650°C) we transformed this metastable metallic a-phase towards its equilibrium semiconducting /3-FeSi z phase composed of large single grains (1 /xm in width). This phase transition was studied both by in-situ techniques such as reflection high energy electron diffraction (RHEED), ultraviolet and X-ray induced photoelectron spectroscopy (UPS and XPS), and ex-situ techniques like transmission electron microscopy (TEM) and scanning Auger microscopy (SAM).

The silicide growth was performed in a MBE vacuum chamber evacuated by a high speed ( ~ 2200 g s -1) turbomolecular pump and large liquid nitrogen cryopanels, leading to a base pressure of ~ 5 X 10-11 torr. The chamber was equipped with both Knudsen cells for sublimating solid sources and various gas introduction lines regulated by electropneumatic valves and calibrated mass flow controllers. High purity disilane (Si2H 6) and iron pentacarbonyl (Fe(CO) 5) were used as precursor gases (gas sources) for deposit-

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Fig. 1. T E M micrographs of (a) the a-FeSi z film grown by CBE at 550°C (plane view). T h e whole film is composed of n u m e r o u s fine a-grains ( ~ 200 A in width) as testified by the corresponding tapir6 fringes. (b) Cross sectional view of the same a-FeSi 2 film. (c) T h e same sample after transformation to a/3-FeSi 2 film by post-annealing up to 650°C. All fine a-grains have coalesced into one single grain with moir~ fringes corresponding to (110)/3-FeSi 2 epitaxially grown on ( l I D Si (plane view). (d) Cross sectional view of the fl-FeSi 2 film.

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J.Y. Natoli et al. / Journal of Crystal Growth 146 (1995) 444-448

ing Si and Fe atoms, respectively. During the CBE process, the Si substrate (2 inch diameter wafer) was maintained at ~ 550°C on its rotating and heating sample holder. In situ and real time m e a s u r e m e n t s of the diffraction patterns of the growing sample were recorded thanks to a CCD camera placed in front of the R H E E D screen and connected to a microcomputer. R H E E D intensity oscillations were also used to cross check the deposition rate together with the mass flow controllers and ion gauge measurements. This M B E - C B E chamber was in situ connected to a multitechnique surface analysis chamber where UPS, XPS, low energy electron diffraction ( L E E D ) and Auger spectroscopy facilities were available. At various stages of the growth, samples could be transferred from the M B E - C B E chamber to the analysis chamber in order to check their physico-chemical properties (cleanliness, elemental composition, stoichiometry of the formed phases, electronic band structure . . . . ). Samples were then p r e p a r e d for ex-situ T E M observation and SAM analysis.

3. Results

With partial pressures of Si2H 6 and Fe (CO) 5 of about ~ 1 × 10 -4 and ~ 4 × 10 - s Torr, respectively, directed onto the S i ( l l l ) substrate maintained at ~ 550°C, we achieved the reproducible growth of a-FeSi 2 films, as testified by in-situ R H E E D m e a s u r e m e n t s (not shown here). Figs. l a and l b show T E M pictures (plane and cross sectional view, respectively) of such an aFeSi 2 film which was indeed composed mostly with fine grains ( ~ 200 .& dimensions) epitaxially grown on S i ( l l l ) as testified by the observed moir6 fringes and T E M diffraction. Due to the a-plane pseudo-ternary symmetry, several epitaxial variants could be observed in agreement with previous studies [14,15]. No contaminant was detected in the XPS spectrum recorded in-situ on the as-grown a-FeSi 2 film, confirming the ability of our CBE technique. Moreover the stoichiometry of the as-grown film was found to correspond to a FeSi 2 phase using the F e 3 p and Si2p core level intensity ratio measured with the XPS tech-

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nique. O t h e r measurements (Auger spectroscopy, UPS) were also consistent with the XPS ones and the a - p h a s e was found to be a metallic material. Figs. 2a and 2b (solid lines) show the XPS Si 2p core level spectrum and UPS valence band spectrum recorded on such an a-FeSi 2 film. The Si2p core level displays an asymmetric tail (arrow at the high binding energy side) [16] suggesting a metallic environment. The UPS valence band unambiguously confirms the a-phase metallic character since a high density Fermi distribution-like peak is observed at the Fermi level E F. With the CBE technique we therefore achieve the unexpected a-FeSi 2 stabilization at a low temperature as recently discovered with R D E [14] and alloepitaxy [11]. We confirm also the epitaxial relationships determined previously with R H E E D and X-ray glancing incidence diffraction [14] which are: (112) a-FeSi 2 planes II (111) Si planes with [201] a-FeSi 2 axis aligned along the [110] Si one. Other variants are also observed [15]. The growth of the tetragonal a - p h a s e thermodynamically unstable at this low t e m p e r a t u r e growth may be attributed to the strain effects of

J.E Natoli et aL /Journal of Crystal Growth 146 (1995) 444-448

Fig. 3. Sequential R H E E D patterns recorded along (110) and (112) Si azimuths during the a - / 3 phase transition with annealing temperature. (a), (b) At 600°C the as-grown rough a-film is smoothing by coalescence as observed with the elongation of the initial R H E E D spots. (c), (d) At 620°C the whole rough a-film is transforming to a smooth /3-film since new diffraction lines are appearing. (e), (f) At 650°C the a - / 3 phase transition is now completed, a features are replaced by/3 features. Note that along the (110) Si azimuth the two phases may be confused because of a similarity in the lattice parameters but they are definitively distinguished along the (112) Si azimuth where additional /3 diffraction lines are observed.

the Si substrate and the favourable lower mismatches with the Si substrate ( ~ 0.8% along the [110] Si axis and ~ 3.9% along the []12] Si axis) as compared to those of the stable orthorhombic /3-FeSi 2 (~ 1.5% and 5.3%, respectively). Keeping in mind that the final aim of the study was to obtain the stable semiconducting/3-phase, we then post-annealed the as-grown a-phase, under ultrahigh vacuum (UHV) environment with a linear ramp of temperature increasing from ~ 550°C up to ~650°C with a rate of 2°C per minute. During this post-annealing, RHEED patterns (Fig. 3) were recorded in real time in order to follow the phase transition. At ~ 620°C the rough a-layer was smoothing; its 3D spotty diffraction pattern was replaced by a 2D streaky one. At ~ 630°C additional streaks characteristic of the /3-phase were appearing and definitively well formed at ~ 650°C. These streaks testify the (110) and (101)/3-planes epitaxial growth on top of the (111) Si planes, as already extensively demonstrated elsewhere [4,17,18]. Fig. 4b displays the variation with increasing temperature of the RHEED intensity profile of one of those/3-characteristic streaks along the (112) Si azimuth (Fig. 3) [4]. The area of this peak is reported in Fig. 4a versus the annealing time (annealing tempera-

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Fig. 4. Real time R H E E D study of the a-fl phase transition. (b) Plot of the R H E E D intensity profile of one characteristic /3 diffraction line along the (112) Si azimuth (as in Fig. 3) versus annealing temperature. The/3 phase is clearly observed at ~ 620°C. (a) The normalized area of this peak is plotted versus annealing time. The sample was annealed with a temperature-programmed heater with a 2°C per minute linearly increasing ramp starting from 550 up to 650°C. Then the temperature was maintained constant with time at 650°C.

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J.Y. Natoli et al. /Journal of Crystal Growth 146 (1995) 444-448

ture). The a-/3 phase transition is clearly demonstrated in this figure. After this transformation, in-situ XPS measurements confirmed that the stoichiometry was still FeSi 2 except a slight Si enrichment at the surface [19]. Figs. 2a and 2b (dotted lines) show the Si 2p core level and valence band spectrum recorded after annealing to obtain the a-fl transition. The Si2p spectrum (Fig. 2a) displays now a more symmetrical tail for a semiconducting environment [16] as compared to the metallic a-one. Consequently, at the Fermi level E r in the valence band spectrum, (Fig. 2b), a lower density of states characteristic of a semiconductor is observed. The/3-film was then examined ex-situ by TEM which confirmed its nature and its epitaxial relationships as already extensively reported previously [2,15,18]. Actually both TEM plane view and cross section images (Figs. lc and ld) revealed a tremendous coalescence of numerous fine a-grains into a large /3-grain during the phase transition, leading to a well oriented single /3-grain the dimension ( < 1 /~m) and the structural high quality of which might be suitable for potential applications. It is worth mentioning here that the thickness ( ~ 100 .A) of this 0-film may be furthermore increased since with CBE technique it is possible to grow directly at 650°C, on top of such a /3-FeSi 2, another /3-film without severely degrading its crystalline purity.

4. Summary In summary we have shown here that the CBE technique is available to epitaxially grow a high quality semiconducting silicide film on silicon via a metastable-stable phase transition. Thanks to in-situ and ex-situ surface and bulk characterization techniques, phase transitions in solid state, involving both structural and electronic properties have been revealed in very thin films and heterostructures.

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