A comparative study on microstructures of β-FeSi2 and carbon-doped β-Fe(Si,C)2 films by transmission electron microscopy

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Abstract

The microstructures of β-FeSi2 and carbon-doped β-Fe(C,Si)2 films, synthesized on Si substrates by metal vapor vacuum arc ion source ion implantor, are studied by transmission electron microscopy. Structure evolution at different annealing temperatures shows that, in comparison with undoped films, the C-doped film quality is improved as manifested by smooth β/Si interface, homogeneous film thickness, fine grains, and high thermal stability. When the energy and dosage are 60 KV and 4×1017 ions/cm2, amorphous layer is formed directly, which transforms into homogeneous and smooth β-FeSi2 surface films after crystallization.

Introduction

Modern microelectronic technology is based on Si materials. Due to its technical compatibility, silicides are attractive materials for use in microelectronic devices. Among them, β-FeSi2 is one of the few semiconductor type silicides with narrow direct band gap of 0.85–0.89 eV. Plus its cost effective and high stability, this material has potential in microelectronic devices [1], [2], in solar cells, and in integrated 1.5 μm wavelength light switch [3], [4], [5], [6], [7].

Many methods have been employed for obtaining high quality thin films grown on Si substrate. Yang et al. [8], [9], Daraktchieva et al. [10], and Li et al. [11] utilized ion beam methods plus post heat treatment for preparing buried and surface β-FeSi2 films. However all the attempts failed in obtaining high-quality films. According to our previous investigation [12], [13], [14], [15], the basic and intrinsic reason is the existence of several competing and non-coherent orientation relationships between β-FeSi2 and Si. Therefore the reduction of lattice mismatch should be a promising way to high quality β-FeSi2 films, and this can be achieved by doping a third element in the β-FeSi2 structure. Doping of the β phase has been investigated by Takakura et al. [16] and Tatsuo et al. [17], who have respectively doped the β phase with Mn and Ru, Ge. No doping using the group IV elements has been studied. We have doped the ion beam synthesized Fe–Si films with C [18]. The reason for choosing C is that C is expected to modify the lattice constants of β-Fe(Si,C)2 structure, without altering the semiconductor properties of the β films. Ion implantation has particular advantages with this respect because the C doping can be done within the same implantation process.

The present paper reports on a comparative study of the microstructure of C-doped and undoped β-type films synthesized by ion implantation.

Section snippets

Experiments

Ion implantation was carried out in metal vapor vacuum arc ion source (MEVVA) 80–10 (maximal accelerating voltage 80 KV, current 10 mA) type ion implantor on single crystal Si(1 0 0) wafers. The implantation parameters are listed in Table 1. Fe and Carbon are consecutively implanted. The vacuum is maintained at about 2×10−6 Torr during implantation and the substrate temperature does not exceed 300 °C. The as-implanted samples were post-annealed at 500, 600, 700 and 850 °C respectively for 1 h. A

Results and discussion

Sample 1 is implanted with Fe under 60 KV and 4×1017 ions/cm2 conditions. Fig. 1(a) shows the cross-section TEM image of the as-implanted state, the viewing direction being perpendicular to Si(1 1 0). A double-layered amorphous film is formed. From the plane-view image shown in Fig. 2(a), the outer layer, with thickness of 50 nm, is a mixture of nanograins imbedded in amorphous matrix. These nanograins have β-type structure as evidenced by the electron diffraction pattern in Fig. 2(b). Fig. 2(c)

Conclusions

The C doping in β-FeSi2 film synthesized by ion implantation improves the film quality as manifested by smoother β/Si interface, more uniform film thickness, finer grains, and higher thermal stability. When the energy and dosage are 60 KV and 4×1017 ions/cm2, amorphous layer is formed directly in the as-implanted state, which transforms into homogeneous and smooth β-FeSi2 surface films after crystallization.

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