C.N McKinty

Amorphous-iron disilicide: A promising semiconductor

We report here the synthesis and the measurements of the microstructural and optical properties of a promising semiconductor, amorphous-iron disilicide. The material was obtained by ion-beam mixing of Fe layers on Si, with Ar8+ ions, at 300 °C. Optical absorption measurements indicate a semiconductor with a direct band gap of 0.88 eV. The significance of this discovery is that it demonstrates the existence of such a material. It should be possible to synthesize by other techniques and could be applied in large-area electronics.

Ion beam synthesized silicides : Growth, characterization and devices

Semiconducting silicides promise particular advantages for the development of optoelectronic devices in silicon. In particular, the direct gap of some of these silicides and the strong optical transitions make them good candidates for efficient light sources in silicon. Our work on the fabrication of iron disilicide and diruthenium trisilicide, materials and devices, using ion beam synthesis, is described here and offers a technology closely compatible with conventional silicon processing.

The optical properties of β-FeSi2 fabricated by ion beam assisted sputtering

β-FeSi2 has been shown to have a minimum direct band gap of 0.87 eV [T.D. Hunt, K.J. Reeson, K.P. Homewood, S.W. Teon, R.M. Gwilliam, B.J. Sealy, Nucl. Instr. and Meth. B 84 (1994) 168–171] which leads to the opportunity for Si based opto-electronics, optical communications and optical interconnects. Electroluminescence has been reported from structures containing β-FeSi2, which were produced by high dose ion implantation and annealing [D. Leong, M.A. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (12 June 1987) 686]. In this paper we report the formation of β-FeSi2 by ion beam assisted co-sputtering of Fe and Si in varying percentages. The layers were deposited with a varying Fe/Si ratio, with a Si capping layer applied to prevent oxidation. Separate regions of the sample were investigated at room temperature using optical absorption, to measure the band gap values. Absorption under the fundamental edge was also analysed at room temperature. Further investigations looked at the temperature dependence of the band gap and the absorption under the fundamental edge. The results showed that a variety of Fe/Si ratios produced β-FeSi2, the formation of which was ascertained by the presence of a suitable band gap value [0.83–0.88 eV]. Absorption under the fundamental edge was shown to follow an exponential Urbach tail [C.H. Grein, S. John, Phys. Rev. B 39 (1989) 1140]. The temperature measurements are in good agreement with the Einstein model.

Ion beam synthesized Ru2Si3

In this letter we report the synthesis of the semiconductor Ru2Si3 by ion implantation into a silicon substrate. The formation of this compound has been confirmed by x-ray measurements and electron diffraction. The absorption coefficient has been determined directly by optical transmission measurements. The band gap is found to be direct with a value in the region of 0.9 eV.

The properties of β-FeSi2 fabricated by ion beam assisted deposition as a function of annealing conditions for use in solar cell applications

In this paper we investigate the formation of b-FeSi2 (from co-deposited layers of Fe and Si produced by ion beam assisted deposition) under a number of annealing regimes (annealing temperatures between 100 and 900 C and times up to 18 h) by optical characterisation of the band edge parameters. The results have indicated that both annealing temperatures and times have a strong effect on the number of defects underneath the fundamental edge of absorption. The measurement temperature dependency of the band gap is also found to be dependent on the annealing conditions. 2002 Published by Elsevier Science B.V.

Properties of β-FeSi2 grown by combined ion irradiation and annealing of Fe/Si bilayers

This paper presents a study of the synthesis of β-FeSi2 layers by irradiation of Fe/Si bilayers with Fe+ ions and a consequent thermal processing, and the analysis of their properties. Characterization of samples included RBS, XRD, cross-section HRTEM, and photo-absorption measurements. It was found that homogenous, highly ordered 275 nm β-FeSi2 layers can be grown by implantation of 90 nm Fe on Si, with 200 keV Fe+ ions at 500°C, followed by 3.5 h annealing at 870°C.

Effect of implant conditions on the optical and structural properties of abeta;-FeSi 2

Semiconducting precipitates of β-FeSi2 have been successfully fabricated in silicon by high dose Fe+ implantation (typically 1.5 × 1016 Fe cm-2 at 200keV). Room temperature electroluminescence (EL) at 1.5μm has been observed from light emitting diodes (LEDs) incorporating this type of structure. This study is to evaluate how the microstructure and optical properties are affected by the implantation parameters, in particular the role of implantation temperature, when high beam current densities are being used. This was done in order to evaluate whether the implant period could be reduced to a commercially realistic time without adversely affecting the optical properties. In this study the implantation temperature was varied and the resulting structures investigated (before and after annealing) using optical absorption, Fourier Transform Infrared Spectroscopy (FTIR), Rutherford backscattering spectroscopy (RBS) and cross sectional transmission electron microscopy (XTEM). A 70 meV decrease in the optical band gap was observed between a sample implanted at 250°C and one implanted at 550°C, a shift in the FTIR spectrum was also observed. RBS and XTEM measurements showed that this change was associated with a change from a surface to a buried silicide layer, with the latter also exhibiting room temperature EL.

Light-emitting diodes fabricated in silicon/iron disilicide

Attempts to obtain electroluminescence from silicon-based devices have been largely frustrated by the indirect bandgap of the semiconductor. One approach, described here, is to fabricate a direct bandgap material which is compatible with silicon processing and which can then be excited via standard carrier injection across p-n junctions. We have used ion implantation of iron, typically at an energy of 180 keV and a dose of 1.5 X 1016 cm-2, conditions which are easily achievable in modern commercial implanters, to form precipitates of (beta) -iron disilicide, which has a direct bandgap of 0.8 eV. At 80 K and under forward bias conditions, the devices emit light at 1.5 micrometers with an external quantum efficiency of 5 X 10-3, and emission at room temperature has been observed. The emission lifetime has been placed at shorter than 60 ns, as expected of a direct bandgap material. Results will be presented showing how the electroluminescence properties change with the dose of implanted iron.