A.K Kewell

Electroluminescence of β-FeSi2 Light Emitting Devices

Ion beam synthesised β-FeSi2 light emitting devices have been fabricated by ion implantation of iron into pre-grown abrupt silicon p–n junctions. Several samples were fabricated by varying the implant conditions and the junction characteristics (layer thickness and doping concentration). Light emission at ~1.5 µm was obtained from all devices but the intensity decreased with increasing temperature. The electroluminescence quenching was found to depend on both the iron implant conditions and the characteristics of the p–n junction.

Is there a future for semiconducting silicides? (invited)

Abstract Silicon is commercially by far the most important semiconductor, however, because silicon has an indirect band gap it would initially appear to be unsuitable for optoelectronic applications. A major research challenge is, therefore, to achieve high intensity light emission from silicon and to engineer active and passive optical structures within it. This paper examines the potential of semiconducting silicides (principally, βFeSi 2 and Ru 2 Si 3 ) for silicon-based optoelectronic applications. It traces the history of the subject from the first photoluminescence spectrum from βFeSi 2 to a working LED which uses βFeSi 2 precipitates as a route for fast radiative recombination. Recent results on semiconducting Ru 2 Si 3 are also reported, which show, for the first time, that this material can be fabricated by high dose ion implantation. They also reveal a direct band gap of 0.91 eV. The future for semiconducting silicides is examined and, although there are still barriers to overcome — the future looks bright.

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.

Electrical, electronic and optical characterisation of ion beam synthesised β-FeSi2 light emitting devices

Abstract β-FeSi2/Si light emitting devices (LEDs) have been attracting great interest since the first successful demonstration of an ion beam synthesised (IBS) device operating at a wavelength of 1.5 μm . We report here on a study of the electrical, electronic and optical properties of devices produced by Fe implantation into epitaxial silicon layers. The devices have been characterised by current–voltage (I–V), capacitance–voltage (C–V), deep level transient spectroscopy (DLTS) and electroluminescence (EL) measurements. DLTS showed the presence of a majority carrier trapping centre, with an activation energy of 200±25 meV. Room temperature EL was observed from β-FeSi2/Si LEDs. Preliminary analysis of the EL results suggests its quench ratio depends on device structure; the quenching is thought to be related to surface recombination.

Waveguides and modulators in 3C-SiC

We have designed and fabricated waveguide optical modulators using cubic silicon carbide-(3C-SiC)-on-insulator rib waveguides. A refractive index change is induced in the rib via the plasma dispersion effect. These types of devices have potential for relatively high-speed silicon-based photonics compatible with silicon processing technology, as compared to pure silicon. Furthermore, the wide bandgap (2.2 eV) of 3C-SiC makes the devices suitable for use over the visible and near infrared spectrum range as well as the longer communication wavelengths. We have demonstrated waveguiding in 3C-SiC, fabricating the waveguides by ion implantation of oxygen into a silicon carbide layer. We have also established a processing recipe for the SiC wafers which enables fabrication of 3- dimensional devices. The work reported here describes the fabrication of the devices and presents preliminary experimental results for the waveguide losses and the modulation of the refractive index as a function of applied current. An efficient waveguide modulator for a single polarization is reported.

Fabrication and evaluation of SiC optical modulators

Silicon Carbide is a potentially useful compound for use in silicon based photonics because cubic silicon carbide (3C- SiC), possesses a first order electro-optic (Pockels) effect, something absent in pure silicon. This means the material is potentially suitable for high speed optical modulation. Furthermore, the wide bandgap (2.2 eV) of 3C-SiC makes the devices suitable for use over the visible and near infrared spectrum range as well as the longer communication wavelengths, and also means the material can tolerate high temperatures. However, relatively little work has been carried out in SiC for photonics applications. In this paper we will discuss design and fabrication of both SiC waveguides and modulators for silicon based photonics. The fabrication process utilizes ion implantation of oxygen into SiC to form the lower waveguide boundary. Subsequently, ribs are etched and contacts are added to form the optical modulators. Consideration of both Pockels modulators and plasma dispersion modulators has been made, and both will be discussed here. These devices have potential for optical modulation, but are also compatible with silicon processing technology. We have demonstrated waveguiding in 3C-SiC, established a processing recipe for the SiC wafers which enables fabrication of 3-dimensional devices, and demonstrated optical modulation. Performance of the resultant devices is compared to other silicon based devices in terms of operating speed and efficiency.

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.