K.J. Reeson

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.

Fabrication of buried layers of SiO2 and Si3N4 a using ion beam synthesis

The properties of buried layers produced by the implantation of high doses of energetic light ions O+ and N+ are reviewed. Similarities and differences in the as implanted and annealed structures are discussed as are the effects of variations in the anneal temperature, time and ramp-rate. To assess the factors governing the interfacial displacements of the growing buried layer with respect to the silicon, experiments utilising buried marker layers and involving sequential implants of O+ or N+ ions were undertaken. These confirmed the results of 18O+ and 15N+ tracer experiments by showing that during implantation the back interface of the buried layer remains effectively fixed while the front interface is displaced towards the silicon surface. During the subsequent anneal redistribution of the implanted species occurs but again this effect is most marked at the front interface.

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.

The role of implantation temperature and dose in the control of the microstructure of SIMOX structures

Abstract Single-crystal ⇇100↩ silicon wafers have been implanted with 200 keV oxygen ions over a dose range of 0.1×10 18 O + cm -2 to 1.4×10 18 O + cm -2 and a temperature range of ≈250°C to 550°C. The specimens have been analyzed, both before and after high-temperature annealing, using a variety of techniques, such as cross-sectional and planar Transmission Electron Microscopy (TEM), Rutherford backscattering (RBS), and ion channelling, Secondary Ion Mass Spectroscopy (SIMS), Infra-red Spectroscopy (IR), and Raman Spectroscopy. This has enabled us to evaluate the development of the SIMOX structure both with respect to implantation temperature and dose and also with respect to annealing temperature and time.

Ion beam synthesis of thin buried layers of SiO2 in silicon

New experiments are reported which explore the possibility of using ion implantation to form thin (<2000 A) buried layers of stoichiometric SiO2 in single crystal silicon, Silicon (100) wafers have been implanted with O+ ions within the dose range 0.1×1018–1.8×1018O+ cm−2 at a particle energy of 200 keV and a substrate temperature of 500°C. Both (100) channelled and non-channelled implants have been carried out. Samples were subsequently annealed at temperatures of up to 1405°C, which causes the oxygen to segregate near the peak of the implanted oxygen distribution. The high dose samples have a continuous buried oxide layer whose thickness scales with the implanted dose, whilst in samples implanted with 0.1×1018O cm−2, discrete, strain free polyhedral precipitates, of diameter 500–1600 A, grow within the single crystal silicon matrix, by a mechanism which is qualitatively similar to oxygen precipitation in C-Z bulk silicon.

Mechanism of buried β-SiC formation by implanted carbon in silicon

Abstract The structure of silicon implanted with high doses of carbon ions in the range (0.35–1.3) × 10 18 C + cm −2 at implantation temperatures from 500 to 700 °C is studied by transmission electron microscopy. At an implantation temperature of 700 °C, cubiv (β-SiC) and hexagonal coherent or semicoherent precipitates are formed in the silicon overlayer. Only the cubic form is stable during high temperature annealing. At high implantation temperatures a thin discrete buried layer of β-SiC, with an epitaxial relationship to the silicon substrate, is formed. The quality of this layer is greatly improved after a high temperature annealing as revealed by the translation-type (111) Moire pattern. The β-SiC epitaxial layer consists of adjoined grains on {111} and {100} planes without appreciable coalescence of these adjoining grains. The stability of the β-SiC precipitates is discussed and it is compared with the stability of SiO 2 precipitates which are formed in silicon implanted with oxygen.

Optical properties and phase transformations in α and β iron disilicide layers

Abstract Ion beam synthesis (IBS) has been used to fabricate semiconducting β-FeSi2 and metallic α-Fe0.82Si2. For all of the doses studied a photoluminescence (PL) signal is observed at 1.54 μm. This signal is first seen after annealing at 800°C and increases in intensity with a commensurate decrease in full width half maximum (FWHM) as the anneal temperature is increased up to 920°C. Likewise the intensity increases and FWHM decreases as the anneal time at 920°C is increased up to 18 h. Optical absorption measurements reveal a linear relationship between the square of the absorption coefficient and the incident photon energy, indicating a direct allowed transition from a semiconductor (β-FeSi2) with a band gap of about 0.87 eV. After annealing at 1000°C no PL or absorption is observed in this spectral region; this is because a thicker, conducting layer of α-Fe0.82Si2, containing ~ 18% Fe vacancies has then been formed. If an α-Fe0.82Si2 layer is subsequently annealed below the phase transition temperature (~ 950°C) then the PL signal reappears as the layer is largely reconverted back to the s-phase.

Investigation of the luminescence properties of Si/βFeSi2/Si heterojunction structures fabricated by ion beam synthesis

Abstract Photoluminescence at 1.54 μm has been observed from discontinuous βFeSi 2 layers fabricated by ion beam synthesis in silicon wafers. A minimum full width half maximum (FWHM) value of 3 me V, measured at 5 K, was obtained from a sample implanted to a dose of 2 × 10 17 Fe ions cm −2 at an energy of 200 ke V, after an 18h anneal at 920°C. When the incident energy was increased the FWHM value also increased reflecting the increased residual damage in the sample after annealing. Room-temperature optical absorption measurements indicate a direct band gap of about 0.86–0.88 e V.

Formation of thin silicon films using low energy oxygen ion implantation

Abstract SIMOX (separation by implanted oxygen) is an established technique to produce device worthy silicon-on-insulator structures. Current interest in thin film fully depleted CMOS devices in SIMOX material has placed emphasis on producing silicon overlayers of 100 nm thickness or less. Thin film SIMOX substrates have been prepared using halogen lamps, to preheat and provide background heating during oxygen ion implantation in the relatively low energy range 50–140 keV. The resulting structures have been studied by RBS, cross-sectional TEM and SIMS. This paper reports on the crystalline quality of the silicon overlayers and discusses the viability of low energy oxygen implantation to produce thin film SIMOX structures suitable for VLSI device fabrication.