A Study on the As Redistribution in P-FeSi, Film Prepared by Reactive Deposition Epitaxy

Lianwei Wang, Chenglu Lin, Xiangdong Chen, and Shichang Zou State Key Laborato y of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of

Sciences, Shanghai 200050, P. R. China

Zuyao Zhou and Xiannhuai Liu Ion Beam Laborato y, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, P.R. China

ABSTRACT-In this paper, we report on the As redistribution properties in S-FeSit prepared by reactive deposition epitaxy. Unlike the case of solid phase epitaxy, no snow plough effect has been found. Arsenic atoms become resident in the surface silicide layer, indicating that the diffusion process during the reactive deposition is different from the case of solid phase epitaxy. Further annealing drive the arsenic atoms out of the film.

I INTRODUCTION

Recently, great attention has been paid to P-FeSi,, a semiconducting material with a direct gap of about 0.87eV[l]. Several methods, such as solid phase epitaxy, reactive deposition epitaxy (RDE), molecular beam epitaxy (MBE), co-deposition and ion beam synthesis (IBS) have been employed to synthesize this material. It was reported by Mahan et a1[2, 33 that a p-FeSi, film prepared by RDE was a genuine large-area single crystalline film. Since then, much work has been devoted to the growth mechanism and epitaxial relationships of the film prepared by RDE [4-61. The dopant redistribution behavior during silicide formation is an important topic for microelectronics. Because of different deposition and thermal treatment process, the difhsion behavior of the dopant may be different. The dopant redistribution properties during the formation of p-FeSi, by solid phase epitaxy have been reported by Erlesand et a1[7]. However, to date there is no report on the diffusion behavior during formation of J3-FeSi2 by reactive deposition epitaxy.

I1 EXPERIMENTAL

Silicon wafers, p-type with the orientation of (1 00) and the resistance of 5-8 i2 cm were at frst cleaned using a standard IC procedure. Arsenic ions were implanted into the silicon wafer at an energy and dose of 40keV and 2x 1OI6 /cm2 respectively. According to TRIM94, the projected range is approximately 33nm. Another set of samples for electrical measurements was also implanted with 4OkeV AS+, but to a dose of 5x10" /cm2, the subsequent procedure being similar. The as-implanted wafer was divided into two parts. The deposition of iron was performed in UMS 500P ultrahigh vacuum chamber by electron beam evaporation. The base pressure of the chamber was mbar, and the working pressure was lO-*mbar. High purity iron (99.99%) was evaporated with an electron beam, the deposition rate was monitored by quadruple mass spectroscopy and was about 1 h e c in our experiments. The thickness of the deposited iron film was obtained by means of the quartz crystal oscillator. On one of the parts, the substrate was maintained at 600°C during the deposition, while on the other part, iron was deposited at room temperature. In both cases, the deposited iron thickness was 30 nm. Subsequent annealing was performed in nitrogen ambient. The As redistribution behavior was characterized by Rutherford back-scattering spectroscopy (Rl3S) using a 2 MeV He' beam and a scattering angle of 165". The interfacial junction behavior was determined by automatic spreading resistance measurements.

In this paper, we report on the As redistribution properties in P-FeSi, prepared by reactive deposition epitaxy. For comparison, the As redistribution behavior during the solid phase epitaxial process as well as that in a sample prepared by RDE during the subsequent annealing has been investigated

111. RESULTS AND DISCUSSION

3.1 Redistribution ofAs during solidphase ep i tqprocess

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For comparison, we study the As redistribution in the case of solid phase epitaxy first. It has been well established that in the case of solid phase reaction in Fe-Si system, Si is the predominant diffusion species[S]. Considering the As atom as an important substitute and point defects always being generated near the silicon-silicide interface, this leads to the pile-up of As atoms at the silicon side near the interface, such effect is called “snow plough”. From the RBS spectra, it can be observed that the position of As peak move toward the low energy during the annealing process. Similar results have been obtained by Erlesand et a1 [7]. The As atom accumulated at the silicon side near the interface, so this region may be n+. It is reported that polycrystalline P-FeSi, always behaves as p- type. It can be predicted that there must be a depletion layer at the interfacial region. In fact, in a reference sample (the energy and dose are 40keV and 5x 10” cm-2 respectively, the thickness of Fe 50 nm), the high resistance region has been detected at the interface of P-FeSi,/Si from the spreading resistance measurements (Fig. l), indicating that there exists a p-n junction.

Fig. 1 Spreading resistance of the reference sample deposited at room temperature after 60O0C/2h plus 80O0C/lh annealing.

3.2 Redistribution of As atoms during the deposition on hot substrate

Fig2 shows the comparison of the RI3S spectra between the film deposited at room temperature and at 600°C. Because of the low diffusive constant, we just assume that

there’s no remarkable diffusion or reaction during the deposition at room temperature. It seems that in the case of deposition at high temperature, the position of As atoms does not change a lot. As we know, reactive deposition always accompanies with the reaction between iron and silicon. This can be deduced from the spectra of Si and Fe. At 6OO0C, the diffusion coefficient of Arsenic atoms is still quite small. If the silicon atoms are still the dominant diffusion species, with the promotion of silicide formation, due to the increment of the thickness of silicide, the position of As should move towards the substrate. What we should mention here is that the thickness of the iron film was approximately the same in both deposition.. So one can conclude that during the deposition at high temperature, the possible case is that the iron must be the dominant diffusion species, this results in the residence of As atoms in the silicide layer at the surface.

Fig.2 RBS spectra of the film deposited at different temperature.

3.3 Redistribution of As atom in silicide layer during subsequent annealing

Finally, we present the investigation the annealing behavior of the film directly deposited at high temperature. It has been mentioned that in the case of reactive deposition, the arsenic atoms become resident in the as-formed silicide layer. Fig. 3 shows the RI3S measurements of the film as-deposited at high temperature and after subsequent annealing. The As atoms move towards the surface after annealing, indicating that the solubility of As in silicide is very low.

Spreading resistance measurements revealed no high resistance region at the interface between the silicide layer and silicon substrate. Since the substrate is p-type, the doping of As does not change the carrier properties of silicide. So in iron disilicide, As does not behave as a donor.

722

E 0 \

i? n d 7-

W s’

C M L

As spectra in (a)

200- ; As moving direction

c

.---. . .

CHANNEL

Fig.3 (a) The Rl3S spectra of the sample deposited at 600°C before and after annealing and (b)Arsenic spectra in Fig.3 (a).

IV CONCLUSION

The redistribution properties of As during reactive deposition epitaxial growth of P-FeSi2 have been investigated. Unlike the case of solid phase epitaxy, no snow

plough effect has been found. Arsenic atoms residence at the surface silicide layer, this implies the dominant diffusion species during the reactive deposition be different from the case of solid phase epitaxy. Subsequent annealing of the film deposited at high temperature drives the As atoms out of the silicide layer, indicating low solubility of As in silicide layer.

“b.0’ 0.1 ’ 02‘ 0.3’ 0.4’ b.5

cEfn-l(P4

Fig. 4 deposited at 600°C after annealing at 600°C/2h + 800”C/lh.

Spreading resistance depth profile of the sample

This work is supported by the National Nature Science Foundation China under the grant number 69576036 and the local Natural Sciences Foundation Shanghai under the contract No. 94JC14006.

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