Si heterostructures

..,.. ,..::::.j:.: .,,, Applied Surface North-Holland

Science

.:“’ ..‘: :.

73 (1993) 141-145

. .

applied surface science

Allotaxial growth of epitaxial Si/FeSi,/Si

heterostructures

0. Miiller, S. Mantl, K. Radermacher, H.L. Bay, G. Crecelius, Ch. Dieker and S. Mesters fiir Schicht- und lonmtrrhnik, Fotxhungszmtrum Jiilich, P.O. Box 191.3, W-5242.5 Jiilich, Germany

lnstitut

Received

2Y March

1993; accepted

for publication

18 May I993

We fabricated Si/FeSiz/Si(ll 1) heterostructures by a new method, called molecular beam allotaxy (MBA). Codeposition of silicon and iron at temperatures around 650°C leads to the formation of FeSi, precipitates in the crystalline silicon matrix. The subsequent annealing was performed in two steps. First annealing at temperatures in the range 1150-1170°C resulting in the formation of high quality buried epitaxial metallic cu-FeSiz layers with a mininum yield value of xrnln = 12%. Cross section transmission electron micrographs (XTEM) reveal these films to be continuous with sharp interfaces. The lowest resistivity for ol-FeSiz was p = 225 ,& cm at room temperature. By a second anneal at temperatures below the phase transition temperature the metallic cu-FeSi? was transformed to the semiconducting fi-FeSi,. These first results prove the applicability of MBA for the growth of metallic and semiconducting FeSi, layers in Si

1. Introduction Epitaxial silicides gained a lot of interest in the last years due to their possible applications in microelectronics. Metallic silicides are commonly used for ohmic and Schottky contacts and as recently shown for the fabrication of permeable base transistors [l-31. Presently, semiconducting silicides are investigated in more detail, because of the potential use of new heterostructures in the Si technology. One of the most interesting silicides is FeSi,, which exists in a metallic and a semiconducting phase. The metallic a-FeSi, grows in a tetragonal structure and is thermodynamically stable at temperatures above 950°C. The semiconducting phase, orthorhombic pFeSi 2, indicates interesting optical properties. A direct transition, as found by optical absorption measurements [4,51 and electron loss spectroscopy [6], promises possible optoelectronic device applications in the IR region. In contrast, band structure calculations [7,8] predict an indirect band gap just below the direct transition. Recent absorption measurements indicated the presence of the indirect transition [9,10]. 0169-4332/93/$06.00

0 1993

Elsevier

Science

Pubhshers

Up to now buried epitaxial cy- and P-FeSi, layers could be fabricated only by high dose ion implantation followed by thermal treatments [11,123. In this paper we present for the first time the fabrication of buried epitaxial (Y- and P-FeSi, layers by a new MBE method, called molecular beam allotaxy (MBA) (see S. Mantl et al. [Appl. Surf. Sci. 73 (1993) 1021. This method was first applied successfully on Si/CoSi JSi( 100) heterostructures [13] and successfully reproduced recently [ 141.

2. Experimental The principle idea of this technique is the formation of a size and depth distribution of silicide precipitates embedded in a single crystalline Si matrix followed by a thermal treatment which results in the formation of a continuous buried layer [13]. To perform these experiments we used a MBE system equipped with two electron guns (base pressure of 5 X lo- “’ mbar). The Si and the Fe evaporation rates are monitored by 3 quartz crystal thin film monitors and a quad-

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rupole mass analyzer. Three inch SitI 11) substrates were chemically cleaned followed by a HF dip. Before deposition we heated the substrates at 850°C for 8 min. First a 80 nm thick Si buffer layer was deposited at 750°C with a Si rate of about 0.15 rim/s.. A trapezoidal-shaped iron profile was embedded in the Si matrix by coevaporation of silicon and iron at temperatures between 500 and 700°C. This led to the formation of different F&i?-precipitate profiles in the crystalline silicon depending on the substrate tcmperature. The iron peak-concentration was varied by the variation of the Si/Fe ratio. The precipitate layer was covered by a IO0 to 150 nm thick Si overlayer. During this process the pressure in the MBE system was below 2 X 10-” mbar. In a second process step the samples were annealed in a rapid thermal annealer (RTA) at 1150 to 1170°C for 10 to 30 s. This led to the formation of a continuous cu-FeSi? layer with thicknesses from 120 to 150 nm, depending on the total amount of the deposited Fe. To transform these cu-FeSi, layers to j3-FeSi,. we annealed these samples a second time between 750 and 850°C for 1 to 20 h in a conventional tube furnace (pressure below 1 x lo-” mbarl. The samples are examined by resistivity measurements, Rutherford backscattering (RBS) and

channeling analysis. Cross section transmission electron micrographs (XTEMI of as-grown and annealed samples were performed to characterize the as-grown precipitate profile, the different phases and the layer quality. In addition electron energy loss spectroscopy (EELS) was carried out to identify the different iron silicide phases.

3. Results and discussion

Fig. la shows the RBS (circles) and channeling spectra (crosses) of a (111) oriented sample prepared at 650°C. The trapezoidal Fc depth profile was achieved by a linear increase of the Fe evaporation rate to a maximum rate of 0.02 rim/s followed by a slow decrease. The total amount of the embedded Fe corresponds to a pure iron thickness of _ 30 nm. This leads to a trapezoidal iron profile with a Fe peak-concentration of 23 at%). The profile is covered by a 100 nm thick Si caplayer. The minimum yield value of the Si epitaxial but over-layer is x,,,,,, = 2S%, indicating defective growth. The minimum yield value of the buried F&i, precipitates amounts to xmln = 50%‘. This value is increased by dechanneling of the

(b)

Fig.

I. (a) RBS (circles) and channeling (crosses) spectra of an as-grown sample deposited at 650°C on Sic I I I ). (h) C‘rosasection TEM micrograph of the same sample.

0. Miller et al. / Allotaxial growth of epitaxial Si / FeSi, / Si heterostructures

top-silicon. These channeling results indicate preferential aligned growth of the FeSi, precipitates. For the as-grown samples a point of investigation was the improvement of the crystallinity of the Si overlayer. For this purpose, we varied the iron peak-concentration, which affects presumably the crystallinity. As expected, lower peakconcentrations led to an improvement, whereas higher peak-concentrations deteroriate the epitaxy of the Si and of the FeSi, precipitates. The upper limit, which is defined by the quality of the top-silicon, is lying in the range of about 26%. On the other hand, we need a minimum peak-concentration of about 20% to obtain continuous layers after annealing. The existence of a minimum peak-concentration for the formation of continuous c-y-FeSi, layers was also found for films fabricated by ion implantation, however, its value is much larger (26 at%) [1.5]. The difference might be due to the very different microstructure between the as-grown (see fig. lb) and the as-implanted sample [ 161. Another way to improve the Si crystal quality is to increase the substrate temperature. Fig. lb presents a XTEM micrograph of the as-grown sample, showing large FeSi, precipitates, extending over the whole Fe profile and mainly oriented

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annealed:

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in growth direction. The structure resembles individual CoSi, “columns”, grown in Sic1 11) in the same temperature range [17]. Due to the trapezoidal Fe profile, the FeSiz precipitates show a “cigar shaped” contour in the Si matrix. The formation of these large FeSi, precipitates is in contrast to CoSi, grown by allotaxy, where small precipitates grow in the wings of the Co profile and larger ones in the middle. These CoSi, precipitates coarsen and coalesce into a continuous layer during annealing [18]. In contrast, the micrographs of the FeSi, samples do not show large variation in the size of the precipitates. At elevated temperatures the dimension along the (111) direction of the “cigar shaped” precipitates is somewhat larger as the final thickness of the film. This complicates the coalescence of the precipitates to a continuous flat layer during thermal annealing. Thus, the substrate temperature must be kept below a certain limit in order to avoid the growth of too large precipitates. The phase of the precipitates was investigated by EELS studies, revealing the dominance of a-FeSi,. This is surprising, since the sample was prepared at 650°C which is far below the formation temperature of cu-FeSi, in thermodynamic equilibrium (950°C). In summary, the improvement of the Si crystal

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Fig. 2. (a) RBS and channeling spectra of a buried cu-FeSi, layer after annealing at 1150°C for 10 s. The simulation of the RBS spectra (solid line) indicates a Fe vacancy concentration of about 19%. (b) Cross section TEM micrograph of the same sample.

quality requires high substrate temperatures, whereas diffusion enhances precipitate growth during the deposition process. At elevated temperatures the growth of large “cigar shaped” precipitates sets an upper limit for the deposition temperature. The optimum temperature for Sit1 11) was found to be between 650 and 675°C. In addition, improvement of the Si crystal quality by lowering the Fe peak-concentration is limited by the existence of a minimum peak-concentration of - 20 at%, required for the formation of continuous films. Surprisingly, the phase of the precipitates is predominantly the metallic cu-FeSi, phase. 3.2. cu-FeSi,

layer formation

Fig. 2a shows RBS and channeling spectra of a sample after RTA at 1150°C for 10 s. By transmission electron diffraction (TED), X-ray diffraction (XRD) and electron energy loss spectroscopy (EELS) the buried layer formed by this annealing step was identified to be a-iron disilicide. The solid line represents a RUMP simulation [19] of an about 120 nm thick layer with a composition The trapezoidal iron profile conof Fe,,,,,%,. tracted to a rectangular iron disilicide distribution. indicating the formation of a continuous

layer. The 120 nm thick a-FeSi, layer is covered with a 170 nm Si layer. The minimum yield value X n,l,, = 22% of the Si overlayer indicates that there is no significant improvement of the crystallinity during the high temperature treatment. This shows that the defects, produced during the coevaporation of the Si and the Fe are thermally stable. In contrast, the channeling signal of the cu-FeSi, layer decreases to xrnin = 25% (despite dechanneling in the top-Si), reflecting epitaxial growth of cu-FeSiZ in Sit1 11). The epitaxial relationship of the cu-Fe%, on Sit1 11) is not clear yet. but is a point of further investigations. In order to determine the crystalline quality of the cu-FeSi, by ion channeling, we removed the top-Si by reactive ion etching (RIE), resulting in a surprisingly low xmin = 12%. To our knowledge this is the best crystallinity of a u-FeSi, layer ever observed. Fig. 2b presents a TEM micrograph of this sample, indicating a continuous cr-FeSi, layer with abrupt interfaces. It is apparent, that more steps exist at the top interface than at the substrate interface. This is probably due to an asymmetry in the “cigar shaped” precipitates. Furthermore, the XTEM micrograph reveals defect structures in the top-silicon, predominantly microtwins oriented along the { 111) planes.

500

Channel Fig. 3. (a) RBS and channeling 800°C for 5 h. The simulation

spectra of a buried @FeSi2 layer obtained from a cu-FeSiZ layer through a second annealing step at of the RBS spectra (solid line) indicates stoichiometric P-FeSi2. (b) Cross section TEM micrograph of the same sample.

0. Miiller

et al. / Allotaxial

growth

This result indicates that twinning occurs already during the deposition at these high temperatures. Unfortunately, these twins are highly temperature stable. To suppress these defects a further improvement of preparation parameters is essential. A further indication for the high quality of the buried a-FeSi, layer was given by resistivity measurements. The lowest values we found are between p = 225 and 250 ~0. cm, depending on the thermal treatment. These values are in the same range as for samples fabricated by ion beam synthesis [151. It is not clear yet, how far this value is lowered by the improvement of the crystalline quality.

of epitaxial

Si / Fe.%, / Si heterostructures

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extending over the whole Fe profile. EELS reveals cY-FeSi, to be the dominating phase of these precipitates. For the formation of continuous layers the Fe peak-concentration in the asgrown samples must be in the range between 20% and 26%. In a first annealing step at 1150 to 1170°C for 10 to 20 s we fabricated buried epitaxial metallic a-FeSi, layers. These layers are continuous with sharp interfaces. The good crystallinity is proved by a minimum yield value of xmi,, = 12% and a resistivity between 225 and 250 pLn. cm, depending on the thermal treatment. Semiconducting P-FeSi, layers are produced by a structural phase transition during a second anneal at 800°C for 5 h.

3.3. p-Fe,!%, layer formation In a second annealing procedure the a-FeSi, layers are transformed to P-FeSi,. The phase transformation was completed after a 5 h anneal at 800°C. The RBS and channeling spectra are presented in fig. 3a. The simulation illustrates a stoichiometric P-FeSi, layer with a thickness of 97 nm covered by 225 nm Si. The minimum yield value of the Si overlayer xmln = 23% does not change during the XOo”C anneal. The corresponding value of the j?-FeSi, layer is ,ymin = 95%. The origin of this poor channeling, as compared to ion beam synthesis samples, is not understood as yet, but is a point of further investigations. It might be due to an epitaxial relationship, which does not show a deep channel minimum. Fig. 3b presents the corresponding XTEM micrograph. As in the case of Lu-FeSi, we find rougher top than bottom interfaces. The microstructure of the silitide layer, indicated by the dark contrast in the /3-FeSi,, is not identified so far.

4. Summary In summary, we showed the applicability of a new MBE deposition technique, called molecular beam allotaxy (MBA), for the formation of Si/FeSi,/Si(lll) heterostructures. We determined the optimum substrate temperature region to be between 650 and 675°C for Sic1 11). Deposition at these temperatures results in the formation of large “cigar shaped” FeSi, precipitates,

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