M. A. Lourenço

Design of InGaAs linear graded buffer structures

The relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, has been studied. Simple InGaAs linear‐graded layers on GaAs substrates never reach complete relaxation. The residual strain in these structures produces a dislocation‐free strained top region while the rest of the buffer is nearly completely relaxed through misfit dislocations, as observed by transmission electron microscopy (TEM). This strained top region is analyzed and its thickness compared with theoretical calculations. The effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x‐ray diffraction, TEM, and low temperature photoluminescence, and results compared with predictions of the models. The optical quality of the cap layer improves when its composition is close to the value that matches the lattice parameter of the strained surface of the grade. The design of linear graded buffers having a strain‐free cap layer with high crystalline quality is...

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.

Efficient silicon light emitting diodes made by dislocation engineering

Abstract Efficient silicon-based light emitting diodes have been fabricated using the dislocation engineering method. Crucially this technique uses entirely conventional ULSI processes. The devices were fabricated by conventional low-energy boron implantation into silicon substrates followed by high-temperature annealing, and strong silicon band edge luminescence was observed. Dislocation engineering is also shown to reduce the thermal quenching for other material systems. Dislocation engineered β-FeSi2 and Er light emitting devices were fabricated and room temperature electroluminescence at ∼1.5 μm was observed in both cases.

Light from Si via dislocation loops

We give a brief overview of the development and recent progress of a new technology, silicon dislocation-engineered light-emitting diodes (LEDs). The dislocation-engineering method enables the development of light emitters in conventional silicon technology. Key and probably unique to this approach is its genuine and total compatibility with the standard ultra-large-scale integration (ULSI) process used to produce complex computer chips.

Engineering of boron-induced dislocation loops for efficient room-temperature silicon light-emitting diodes

We have studied the role of boron ion energy in the engineering of dislocation loops for silicon light-emitting diodes (LEDs). Boron ions from 10to80keV were implanted in (100) Si at ambient temperature, to a constant fluence of 1×1015ions∕cm2. After irradiation the samples were annealed for 20min at 950°C by rapid thermal annealing. The samples were analyzed by transmission electron microscopy and Rutherford backscattering spectroscopy. It was found that the applied ion implantation∕thermal processing induces interstitial perfect and faulted dislocation loops in {111} habit planes, with Burgers vectors a∕2⟨110⟩ and a∕3⟨111⟩, respectively. The loops are located around the projected ion range, but stretch in depth approximately to the end of range. Their size and distribution depend strongly on the applied ion energy. In the 10keV boron-implanted samples the loops are shallow, with a mean size of ∼30nm for faulted loops and ∼75nm for perfect loops. Higher energies yield buried, large, and irregularly shape...

Deep level transient spectroscopy of CdS/CdTe thin film solar cells

Deep levels in polycrystalline n-CdS/p-CdTe photovoltaic structures have been studied by deep level transient spectroscopy (DLTS). The results were obtained from cells which have undergone different post-deposition treatment (as-deposited, heat treated, and heat treated in the presence of CdCl2). The DLTS results showed a deep level distribution independent of the post-deposition treatment. For all samples the spectra were dominated by a hole trap, localized at EV+0.48 eV. Metastable hole and electron traps were also observed, mainly in the heat treated (with and without a CdCl2 layer) devices.

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.

On the role of dislocation loops in silicon light emitting diodes

The role of boron-induced dislocation loops on the suppression of the luminescence thermal quenching in silicon-based light-emitting diodes is investigated here. Luminescence measurements and cross-sectional transmission-electron-microscopy images from devices fabricated by boron implantation into crystalline silicon, and into a pre-amorphized substrate, to prevent the boron-induced loops formation, were compared. The results show that, in the devices incorporating dislocation loops between the depletion region and sample surface (the boron induced loops), the thermal quenching has been completely eliminated, in contrast with devices fabricated from the pre-amorphized substrate where strong thermal quenching is still observed.

Extraordinary optical gain from silicon implanted with erbium

Here we report on measurements of optical gain at 1.5μm in crystalline silicon. Gain is achieved by the incorporation of the rare earth erbium in silicon. A method was developed to enable the gain measurement in short silicon waveguides. Crucially, gain values obtained are significantly greater than previously supposed. We have measured a lower limit for the optical cross section for Er3+ of 5×10−19cm2, 30 times higher than previously anticipated. Given these higher values, this system now offers a realistic route to the production of electrically pumped silicon optical amplifier and laser devices using standard silicon process technology.

Silicon light emitting diodes emitting over the 1.2–1.4μm wavelength region in the extended optical communication band

Here, we demonstrate bulk silicon light emitting diodes operating over the 1.2–1.35μm range. This is achieved by the implantation of the rare earth thulium, incorporated in the trivalent Tm3+ state, into silicon p-n junctions. Light emitting diodes operating under forward bias have been obtained by codoping of boron to reduce the thermal quenching. Seven sharp lines are observed, corresponding to known internal Tm3+ transitions in the manifold from the H53 to the H63 ground states. This center, together with the basic 1.15μm silicon emitters and Si:Er devices operating at 1.54μm, now enables significant coverage of the extended (1.1–1.8μm) optical communications band in silicon.