M. Harry

A silicon/iron-disilicide light-emitting diode operating at a wavelength of 1.5 μm

Although silicon has long been the material of choice for most microelectronic applications, it is a poor emitter of light (a consequence of having an ‘indirect’ bandgap), so hampering the development of integrated silicon optoelectronic devices. This problem has motivated numerous attempts to develop silicon-based structures with good light-emission characteristics, particularly at wavelengths (∼1.5 μm) relevant to optical fibre communication. For example, silicon–germanium superlattice structures can result in a material with a pseudo-direct bandgap that emits at ∼1.5 μm, and doping silicon with erbium introduces an internal optical transition having a similar emission wavelength, although neither approach has led to practical devices. In this context, β-iron disilicide has attracted recent interest as an optically active, direct-bandgap material th might be compatible with existing silicon processing technology. Here we report the realization of a light-emitting device operating at 1.5 μm that incorporates β-FeSi2 into a conventional silicon bipolar junction. We argue that this result demonstrates the potential of β-FeSi2 as an important candidate for a silicon-based optoelectronic technology.

Optical absorption study of ion beam synthesized polycrystalline semiconducting FeSi2

Ion beam synthesized polycrystalline semiconducting FeSi2 on Si(001) has been investigated by transmission measurements at temperatures between 10 and 300 K. The existence of a minimum direct band gap was demonstrated and its variation with the temperature was studied by means of a three‐parameter thermodynamic model and the Einstein model. Band tail states and states on a shallow impurity level were found to give rise to the absorption below the fundamental edge. The presence of an Urbach exponential edge was shown and the temperature dependence of the Urbach tail width was also studied based on the Einstein model. A strong structural disorder associated with grain boundaries between and within the FeSi2 grains and their related defects was found to be the dominant contribution at room temperature.

On the origin of the 1.5 μm luminescence in ion beam synthesized β-FeSi2

In this letter we present photoluminescence results on β‐FeSi2/Si using excitation energies above and below the silicon band gap. These results show that the luminescence emission observed at 1.5 μm can be firmly attributed to band edge related emission from the β‐FeSi2. This result confirms the potential of β‐FeSi2 as a strong contender for a silicon compatible optoelectronics technology that matches the conventional optical fiber transmission wavelength at 1.5 μm.

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.

Electrical, optical and materials properties of ion beam synthesised (IBS) FeSi2

The electrical and optical properties of FeSi2 structures produced by ion beam synthesis (IBS) are investigated. Above 150 K both α and βFeSi2n-Si structures display good Schottky diode characteristics. βFeSi2n-Si exhibits a low reverse leakage current up to −20 V after which abrupt avalanche breakdown occurs. As expected, the reverse leakage current of αFeSi2n-Si for the same diode area, is an order to magnitude higher than that for the βFeSi2n-Si diodes and the breakdown is less abrupt. The characteristics of both types of diode suggest that thermionic emission is the main conduction mechanism across the barrier. For samples implanted with higher doses of Fe, where a continuous layer of βFeSi2, is produced, the photoluminescence (PL) signal is indistinguishable from the background noise at 80 K. However, for lower dose samples (in the dose range 5 × 1015−1 × 1017 Fe cm−2) although the peak position remains the same the signal intensity is significantly increased and is visible at 80 K. Cross-sectional transmission electron microscopy (XTEM) results from these samples show precipitates with diameters of 400–600 A at the surface and smaller precipitates <50 A in diameter around the projected range of the implant. For even lower dose samples (1 × 1014 Fe cm−2) no PL signal is observed which correlates to the lack of βFeSi2 precipitates in the XTEM micrographs.

Order domain boundaries in ion beam synthesized semiconducting FeSi2 layers

The internal streaking contrast within ion beam synthesized β‐FeSi2 (β) grains has been studied. The results show that this internal streaking contrast is caused by the interfaces between coexistent β order domains (ODs) which are 90° oriented to one another around [200]β. The interface between adjacent ODs is (200)β. The mechanism for the formation of order domain boundaries (ODBs) is attributed to the impingements of separately nucleated growing silicide nuclei during the process of ion implantation and subsequent thermal annealing.

Structural properties of ion beam synthesized iron - cobalt silicide

Surface and buried layers of ternary silicide were fabricated by implantation of iron and cobalt into (100) silicon wafers. For the surface layers two sets of samples with different iron to cobalt ratios were prepared. In the first set, cobalt was implanted first followed by iron and the implant order was reversed in the second set. In all cases the total implanted dose (Fe+Co) was kept constant. For the buried ternary silicides two samples were prepared with equal doses of iron and cobalt. In the first sample, cobalt was again implanted first with the implant order being reversed for the second sample. The physical properties of the synthesized layers were investigated by Rutherford backscattering spectrometry (RBS), x-ray photoelectron spectroscopy (XPS) and cross sectional transmission electron microscopy (XTEM). The results indicate that the implantation order is critical to the subsequent development of the synthesized layer. The surface layers with iron implanted first were non-crystalline and showed no significant improvement of crystallinity with increasing anneal temperature. However, the surface layers with cobalt implanted first exhibited a large improvement of crystal quality with increasing anneal temperature. Segregation into separate iron- and cobalt-rich layers was also observed for surface layers where cobalt was implanted first for Co:Fe . The crystalline quality of the buried layers was also determined by the implant order, in a similar way to that for the surface layers.

TEM investigation of ion beam synthesized semiconducting FeSi2

Abstract Both as-implanted and annealed semiconducting FeSi 2 samples fabricated using ion beam synthesis technique were studied by transmission electron microscopy. A continuous buried silicide layer with larger precipitates and sharper interfaces formed during annealing at 900 °C for 18 h. Different orientation relationships between the silicide grains and the silicon substrate were found. The existence of a large variety of parallel lattice plane pairs with the available small mismatches between the two phases results in these preferred orientation relationships.

Beam-power heating effect on the synthesis of graded composition epitaxial Si1−xGex alloy layers

Formation of epitaxial Si1−xGex layers by high-dose Ge+ implantation into Si has been investigated under different beam-power densities regimes. Ge+ ions with incident energies of 200 and 250 keV were implanted into (100) Si. Ion fluences were between 0.5 × 1016 and 1 × 1017Ge+/cm2, with current densities varied between 0.2 and 12 μA/cm2. In one set of samples a second amorphizing 500 keV Si+ implant was also performed at low temperature (< −50°C) before Solid Phase Epitaxial (SPE) regrowth. As implanted and thermal processed layers have been studied by cross-sectional Transmission Electron Microscopy (XTEM) and Rutherford Backscattering Spectrometry and ion-Channeling (RBS-C). In the low beam-power density regime (⋍ 0.2 W/cm2) small variations in the current density value have been found to affect dramatically the crystalline quality of the SPE regrown layers. This has been attributed to beam-power heating occurring during the synthesis of the layers which determines the quality of the amorphous/crystalline (a/c) interface and of the crystalline seed for SPE regrowth. The implantation temperature for different beam-power densities has been measured and a method to control the a/c interface quality by using a double-step Ge+ implant process has been demonstrated. Samples implanted with high beam-power densities (⋍ 2 W/cm2) resulted in highly damaged crystalline materials. The high-temperature annealing required to remove the implantation damage is limited by the Ge diffusion, while appreciable recovery of the crystal quality is achieved if a post amorphization process by low temperature Si+ implant is used in conjunction with SPE regrowth.

Ternary iron-cobalt silicide fabricated by ion beam synthesis

Ternary silicide layers were fabricated by the implantation of iron and cobalt into (100) silicon wafers. Two sets of samples with different iron to cobalt ratios were prepared, with cobalt being implanted first followed by iron in the first set, and the implantation order being reversed for the second set. In all cases a total Fe + Co dose of 5 × 1017 cm−2 was used. The structural properties of the synthesised layers were assessed by Rutherford backscattering spectrometry (RBS) and cross-sectional transmission electron microscopy (XTEM). Our results indicate that for the samples where cobalt was implanted first epitaxial layers are formed after implantation. However, when iron was implanted first the layers are non-crystalline. For the samples with iron implanted frrst no significant improvement in crystal quality was observed with increasing anneal temperature. However, for those samples where cobalt was implanted first we observed a strong dependence of the crystal quality on the annealing temperature. For the samples with cobalt doses ≥ 2.5 × 1017 cm−2, where cobalt was implanted first, segregation of iron and cobalt within the synthesised layer is observed.