E. Grilli

Room‐temperature visible luminescence from silicon nanocrystals in silicon implanted SiO2 layers

We report the observation of visible‐light emission at room temperature from high fluence (0.3–3×1017 cm−2) Si+ implanted thermal SiO2 layers grown on silicon substrates. Significant blue‐light emission and an intense broad luminescent band with a peak beyond 750 nm are observed after annealing at high temperature (T≥1000 °C). The red‐light emission, present only in the highest fluence implant, is attributed to the luminescence emitted from silicon nanocrystals produced by silicon precipitation. The presence of silicon nanocrystals is confirmed by transmission electron microscopy. Significant blue‐light emission is visible after thermal annealing in the 1×1017 cm−2 fluence implant. The peak position shifts from 490 to 540 nm by increasing the annealing cycles temperature.

Thermally activated carrier transfer and luminescence line shape in self-organized InAs quantum dots

We investigated the temperature dependence (10–180 K) of the photoluminescence (PL) emission spectrum of self‐organized InAs/GaAs quantum dots grown under different conditions. The temperature dependence of the PL intensity is determined by two thermally activated processes: (i) quenching due to the escape of carriers from the quantum dots and (ii) carrier transfer between dots via wetting layer states. The existence of different dot families is confirmed by the deconvolution of the spectra in gaussian components with full width half maxima of 20–30 meV. The transfer of excitation is responsible for the sigmoidal temperature dependence of the peak energies of undeconvoluted PL bands.

Phonon strain shift coefficients in Si1−xGex alloys

A comprehensive study of the biaxial strain-induced shift of the Si1−xGex Raman active phonon modes is presented. High-resolution Raman measurements of Si1−xGex/Si heterostructures have been compared to x-ray diffraction data. Our approach, unlike previous works, is effective to decouple and quantify separately the effect of strain and composition on the phonon frequencies, yielding an accurate determination of the phonon strain shift coefficients in the entire composition range. Our results show that the strain shift coefficients are independent of the composition, a result which is in good agreement with theoretical calculations, performed within the framework of valence force-field theory.

Low density GaAs∕AlGaAs quantum dots grown by modified droplet epitaxy

Low temperature photoluminescence spectroscopy is used to analyze the effects of the Ga coverage and of the postgrowth thermal annealing on the electronic properties of low density (≈1×109cm−2) self-assembled GaAs∕AlGaAs quantum dots (QDs) grown by modified droplet epitaxy (MDE). We demonstrate that with the MDE method it is possible to obtain low density and high efficiency QD samples with high photoluminescence efficiency. Large modifications of the photoluminescence band, which depend on Ga coverage and thermal annealing, are found and quantitatively interpreted by means of a simple model based on the Al-Ga interdiffusion.

Structural and optical characterization of self-assembled InAs-GaAs quantum dots grown on high index surfaces

The structural and the optical properties of InAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (QDs) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QD coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) spectra of GaAs (100) and (311) samples show typical QD features with PL peaks in the energy range 1.15-1.35 eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70K was observed. These results suggest that the high index substrates are promising candidates for production of high quality self-assembled QD materials for application to photonics.

Room temperature photoluminescence of Ge multiple quantum wells with Ge-rich barriers

We report on the observation of room temperature direct band gap photoluminescence in compressively strained-Ge multiple quantum wells with Ge-rich SiGe barriers. A detailed experimental study of the temperature dependence of the photoluminescence is carried out from 5 K up to room temperature. We find that the direct gap photoluminescence at room temperature is due to the thermal excitation of carriers from L-type to Γ-type confined states. Room temperature photoluminescence shows that Ge/SiGe multiple quantum wells are promising candidates for efficient light emitting devices monolithically integrated on Si.

Room-temperature electroluminescence of ion-beam-synthesized β-FeSi2 precipitates in silicon

A simple silicon-based electroluminescent device has been realized, embedding β-FeSi2 precipitates in the depletion region of a Si p–n junction by ion-beam synthesis, a process fully compatible with microelectronics technologies. Light emission peaked at about 1.6 μm has been observed up to room temperature. The luminescence signal is shown to be due to interband recombination in the crystalline nanoprecipitates.

The photoluminescence emission in the 0.7-0.9 eV range from oxygen precipitates, thermal donors and dislocations in silicon

There is a wide set of literature reports that suggest that over-coordinated oxygen or self-interstitials are, directly or indirectly, the chemical bridge between thermal donors, oxygen precipitates and dislocations, capable of supporting a common origin of their emission features in the 0.7-0.9 eV range. Finding the experimental proof of these suggestions was the aim of this present work, which required both appropriate preparation of samples and their careful optical, electrical and microscopical characterization. We were able to show not only that the photoluminescence emissions from oxide precipitates could be correlated to their density and to the presence of closed dislocation rings around them, but also that the precursors of dislocations are optically active as well. For samples thermally annealed in the range of thermal donors, we were able to show that their optical activity seems to be correlated to a transition from a shallow donor level of thermal donors to a deep level of a CiO2 complex.

Piezoelectric-induced quantum-confined Stark effect in self-assembled InAs quantum dots grown on (N11) GaAs substrates

We investigate the optical properties of InAs self-assembled quantum dots grown on (N11)A/B GaAs substrates, by means of cw photoluminescence under different excitation power densities. We observe a sizeable blue-shift of photoluminescence band induced by increasing the photogenerated carrier density. The shift depends on the substrate orientation and exhibits a strong asymmetric dependence on the substrate termination. We attribute the photoluminescence blue-shift to a reverse quantum confined Stark shift of ground state transition energies in the quantum dots. This effect arises from the photogenerated charge screening of the built-in piezoelectric field present in such strained structures grown on high index planes.

Efficient room temperature carrier trapping in quantum dots by tailoring the wetting layer

The temperature dependence of carrier confinement in states of self-assembled In0.5Ga0.5As quantum dots (QDs) embedded in AlyGa1−yAs barriers has been investigated by means of photoluminescence (PL) measurements. We show that photoexcited carriers above the AlGaAs barriers have two recombination channels that contribute to the temperature quenching of the PL from QDs: (a) carrier losses in the AlGaAs layers during the relaxation process and (b) thermal evaporation of captured carriers out of QDs. The interplay between these two mechanisms determines the behavior of the nonresonantly excited photoluminescence as a function of temperature. Eliminating the first contribution by using resonant excitation of the QD PL, we demonstrate a definite enhancement of the carrier confinement at room temperature in InGaAs/AlGaAs QDs by increasing the Al content. We show that this effect is related to the increase in the energy separation between the electronic states in the QD and the wetting layer.