Modification of β-FeSi2 precipitate layers in silicon by hydrogen implantation

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Abstract

Semiconducting iron disilicide precipitates have been fabricated by ion beam synthesis in silicon as well as in silicon containing a cavity layer prepared by a hydrogen implant and subsequent rapid thermal annealing. The samples were characterised by photoluminescence, Raman spectroscopy and cross section transmission electron microscopy. We find no significant shifts of the FeSi2 Raman lines, indicating that the iron disilicide precipitates are in the same strain state in all our samples. Furthermore, comparing the 246 cm−1 FeSi2 Raman line from precipitates and from a buried polycrystalline layer, we conclude that most of the FeSi2 precipitates are unstrained. Consequently, the variation of the photoluminescence in differently prepared samples cannot be due to strain effects in the silicide precipitates.

Introduction

Semiconducting iron disilicide (β-FeSi2) has been under intense study due to its potential for silicon-based optoelectronics [1], [2]. In recent years, much attention has been given to the study of small precipitates of β-FeSi2 embedded in silicon, especially after the demonstration of room temperature electroluminescence in a device containing β-FeSi2 precipitates [3]. However, it is still controversial whether the light emission in such structures is due to recombination in the silicide or due to extended defects in the silicon surrounding the precipitates [4], [5].

Even though iron disilicide is an indirect gap semiconductor [6], [7], there is theoretical evidence for the possibility of a transition to a direct gap because of strain effects [8], [9]. If the lattice mismatch between iron disilicide and silicon is compensated by strain in the silicide, the material could become direct gap. It has been proposed that this is the physical reason for the efficient light emission from precipitate layers. Very recently, however, evidence has been given that the luminescence can be attributed to large, unstrained precipitates [10]. In this work, we study the strain in iron disilicide precipitates and in the surrounding silicon matrix by Raman spectroscopy. We have compared the position and the width of the 246 cm−1 line typical for β-FeSi2with a buried, polycrystalline β-FeSi2 layer. Furthermore, the 520 cm−1 line coming from the silicon matrix in our samples has been compared with perfect, crystalline silicon.

We have used silicon containing a cavity layer as an alternative host material to modify the strain state of β-FeSi2 precipitates. The cavity layer was fabricated using hydrogen implantation and subsequent rapid thermal annealing. A similar process has been previously used for strain relaxation in Si/SiGe heterostructures [11].

The samples were characterised by photoluminescence to check whether there is a correlation between the strain in the β-FeSi2 precipitates and their luminescence properties.

Section snippets

Experimental

We fabricated iron disilicide precipitates in two hosts: perfect silicon and silicon containing a cavity layer. The cavity layers were prepared as follows. As starting material we used high resistivity, n-type, float zone (100) silicon wafers (ρ>1 kΩ cm). Hydrogen was implanted at room temperature to a dose of 2×1016 H+/cm2, using 10 keV as implant energy. A subsequent rapid thermal annealing at 800°C for 7 min in an argon atmosphere was used to form a cavity layer in a depth of 150 nm

Results and discussion

The photoluminescence spectra of our samples are shown in Fig. 2. The main feature is an intense peak at 0.8 eV, often accompanied by weaker peaks at 0.84, 0.87 and 0.93 eV. There is a broad background component. The exact shape of the spectra is rather sensitive to sample preparation. The luminescence intensities in the precipitate samples A–G vary over an order of magnitude. The luminescence from our reference sample H is weaker by two orders of magnitude compared to sample E. The hydrogen

Conclusion

Semiconducting iron disilicide precipitates in silicon and in a silicon layer containing cavities have been studied by TEM, photoluminescence and Raman spectroscopy. The photoluminescence spectra have shown similar features in all samples, despite large intensity variations. A comparison of the Raman spectra of the samples containing β-FeSi2 precipitates with a buried polycrystalline β-FeSi2 layer provides no evidence for strain. This is in agreement with a TEM analysis published recently [10].

Acknowledgments

We would like to thank St. Lenk for the XTEM micrographs and M. Gebauer and W. Michelsen for the ion implantations.

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