Salvatore Coffa

Room‐temperature electroluminescence from Er‐doped crystalline Si

We have obtained room‐temperature electroluminescence (EL) at ∼1.54 μm from Er and O co‐doped crystalline p‐n Si diodes fabricated by ion implantation, under both forward and reverse bias conditions. Under forward bias, the EL intensity decreases by a factor of ∼15 on going from 110 to 300 K, where a weak peak is still visible. In contrast, we report the first sharp luminescence peak obtained under reverse bias conditions in the breakdown regime. In this case the EL intensity decreases only by a factor of 4 on going from 110 to 300 K and the room‐temperature yield is more than one order of magnitude higher than under forward bias. The data suggest that Er excitation occurs through electron‐hole mediated processes under forward bias and through impact excitation under reverse bias.

The erbium‐impurity interaction and its effects on the 1.54 μm luminescence of Er3+ in crystalline silicon

We have studied the effect of erbium‐impurity interactions on the 1.54 μm luminescence of Er3+ in crystalline Si. Float‐zone and Czochralski‐grown (100) oriented Si wafers were implanted with Er at a total dose of ∼1×1015/cm2. Some samples were also coimplanted with O, C, and F to realize uniform concentrations (up to 1020/cm3) of these impurities in the Er‐doped region. Samples were analyzed by photoluminescence spectroscopy (PL) and electron paramagnetic resonance (EPR). Deep‐level transient spectroscopy (DLTS) was also performed on p‐n diodes implanted with Er at a dose of 6×1011/cm2 and codoped with impurities at a constant concentration of 1×1018/cm3. It was found that impurity codoping reduces the temperature quenching of the PL yield and that this reduction is more marked when the impurity concentration is increased. An EPR spectrum of sharp, anisotropic, lines is obtained for the sample codoped with 1020 O/cm3 but no clear EPR signal is observed without this codoping. The spectrum for the magnetic...

Mechanism and performance of forward and reverse bias electroluminescence at 1.54 μm from Er-doped Si diodes

We have analyzed the mechanisms and the efficiency of the 1.54 μm electroluminescence from Er-doped crystalline Si. Optical doping of a 0.25 μm deep p+−n+ junction was achieved by multiple Er and O implants which realize a uniform concentration of 1019 Er/cm3 and 1020 O/cm3 from 0.2 to 0.9 μm from the surface. It has been found that, for the same current density passing through the device, the room temperature electroluminescence signal is 2–10 times higher under reverse bias at the diode breakdown than under forward bias. Detailed analyses of the spectrum line shape, temperature, and current density dependencies and modulation performances under both forward and reverse bias allowed us to elucidate the reasons for this difference. In forward bias, in spite of the large effective excitation cross section (>6×10−17 cm2 at 300 K), the efficiency of room temperature electroluminescence is limited by the small number of excitable sites (∼1% of the total Er concentration) and by the efficiency of nonradiative ...

Evolution from point to extended defects in ion implanted silicon

We present a quantitative study of the evolution of point defects into clusters and extended defects in ion-implanted Si. Deep level transient spectroscopy (DLTS) measurements are used to identify and count the electrically active defects in the damaged region produced by Si ion implantation at energies of 145 keV–2 MeV, and fluences from 1×108 to 5×1013 Si/cm2. Analyses of silicon annealed in the temperature range 100–680 °C allow us to monitor the transition from simple point defects to defect clusters and extended defects that occur upon increasing the ion fluence and the annealing temperature. At low doses, <1010 Si/cm2, only about 2% of the Frenkel pairs generated by the ion beam escape recombination and are stored into an equal number of interstitial and vacancy-type point defects. Thermal treatments produce a concomitant annealing of interstitial and vacancy-type defects until, at temperatures above 350 °C, only two to three interstitial-type defects per ion are left, and the DLTS spectra contain s...

Si-based materials and devices for light emission in silicon

Abstract We report on the fabrication and performances of extremely efficient Si-based light sources. The devices consist of MOS structures with erbium (Er) implanted in the thin gate oxide. The devices exhibit strong 1.54 μm electroluminescence (EL) at 300 K with a 10% external quantum efficiency, comparable to that of standard light-emitting diodes using III–V semiconductors. Er excitation is caused by hot electrons impact and oxide wearout limits the reliability of the devices. Much more stable light-emitting MOS devices have been fabricated using Er-doped silicon rich oxide (SRO) films as gate dielectric. These devices show a high stability, with an external quantum efficiency reduced to 1%. In these devices, Er pumping occurs by energy transfer from the Si nanostructures to the rare-earth ions. Finally, we have also fabricated MOS structures with Tb- and Yb-doped SiO 2 which show room temperature EL at 540 nm (Tb) and 980 nm (Yb) with an external quantum efficiency of a 10% and 0.1%, respectively.

High efficiency and fast modulation of Er‐doped light emitting Si diodes

We demonstrate that the electrical excitation of Er ions incorporated within the depletion layer of a p+−n+ Si diode allows one to simultaneously obtain efficient pumping of rare earth ions and a fast turnoff time of the electroluminescence signal. In fact it is found that during pumping, under reverse bias at the breakdown, a high internal quantum efficiency (10−4) can be achieved since the Er ions are excited with a cross section of 6×10−17 cm2 and exhibit a decay lifetime of 100 μs at room temperature. On the other hand, when the diode is turned off, the electroluminescence signal dies off in less than 10 μs (a limit set by the time response of the adopted detector). These results are explained by observing that fast nonradiative decay of the excited Er ions can occur by Auger transfer of the energy to a free electron or to an electron bound to an Er‐related level in the bandgap. These processes are inhibited within the depletion layer and only set in when, at the turnoff, the excited Er ions are sudde...

Light Emission From Er-Doped Si: Materials Properties, Mechanisms, and Device Performance

The achievement of efficient room-temperature light emission from crystalline Si is a crucial step toward the achievement of fully Si-based optoelec-tronics. However Si, the leading semiconductor in microelectronic applications, is unable to perform as well in the optical arena. In fact due to its indirect bandgap, Si does not exhibit efficient light emission and has been considered unsuitable for optoelectronic applications. Several efforts have been dedicated to overcoming this limitation. Among them, luminescence through the incorporation of rare-earth impurities has been considered In particular, erbium doping has been demonstrated as a valid approach toward achievement of efficient light emission from Si. 1−43 Erbium is a rare-earth ion that, in its 3+ state, can emit photons at 1.54 μ m because of an intra-4 f shell transition between the first excited state ( 4 I 13/2 ) and the ground state ( 4 I 15/2 ). This emission is particularly attractive because its wavelength falls inside a window of maximum transmission for the silica optical fibers. When Er ions are inserted within a Si matrix, the excitation ( 4 I 15/2 → 4 I 13/2 ) can be achieved through the carriers provided by the host, whereas the subsequent deexcitation ( 4 I 13/2 → 4 I 15/2 ) can result in a sharp, atomlike light emission.

The effects of oxygen and defects on the deep-level properties of Er in crystalline Si

We have investigated the electronic properties of Er in crystalline Si using deep‐level transient spectroscopy and capacitance‐voltage measurements. Erbium was incorporated by ion implantation in a p+‐n junction structure. In order to explore the role of oxygen and defects some samples were coimplanted with O and the annealing behavior of the deep‐level spectra was explored in the temperature range 800–1000 °C for annealing times ranging from 5 s to 30 min. We show that O‐codoping produces large modifications in the Er‐related deep‐level spectra and, in particular, a promotion from deep to shallow levels, thus enhancing the donor behavior of Er in Si. For erbium implanted in pure crystalline Si the spectrum is dominated by deep levels arising from Er‐defect complexes which are easily dissociated upon thermal annealing. In O‐coimplanted samples the formation of Er‐O complexes with a characteristic level at EC−0.15 eV is observed. These complexes form upon thermal annealing and are stable up to 900 °C. Thes...

High sensitivity protein assays on microarray silicon slides.

In this work, we report on the improvement of microarray sensitivity provided by a crystalline silicon substrate coated with thermal silicon oxide functionalized by a polymeric coating. The improvement is intended for experimental procedures and instrumentations typically involved in microarray technology, such as fluorescence labeling and a confocal laser scanning apparatus. The optimized layer of thermally grown silicon oxide (SiO(2)) of a highly reproducible thickness, low roughness, and fluorescence background provides fluorescence intensification due to the constructive interference between the incident and reflected waves of the fluorescence radiation. The oxide surface is coated by a copolymer of N,N-dimethylacrylamide, N-acryloyloxysuccinimide, and 3-(trimethoxysilyl)propyl methacrylate, copoly(DMA-NAS-MAPS), which forms, by a simple and robust procedure, a functional nanometric film. The polymeric coating with a thickness that does not appreciably alter the optical properties of the silicon oxide confers to the slides optimal binding specificity leading to a high signal-to-noise ratio. The present work aims to demonstrate the great potential that exists by combining an optimized reflective substrate with a high performance surface chemistry. Moreover, the techniques chosen for both the substrate and surface chemistry are simple, inexpensive, and amenable to mass production. The present application highlights their potential use for diagnostic applications of real clinical relevance. The coated silicon slides, tested in protein and peptide microarrays for detection of specific antibodies, lead to a 5-10-fold enhancement of the fluorescence signals in comparison to glass slides.

Direct evidence of impact excitation and spatial profiling of excited Er in light emitting Si diodes

We provide direct evidence that Er ions incorporated in the depletion layer of a p+–n+ Si junction are efficiently pumped through an impact excitation process with hot carriers. The carriers were accelerated by the electric field present in the depletion layer after being produced by either Zener breakdown of the junction at ∼5 V or by irradiating the diode with an argon laser. Measurements of the electroluminescence yield at 1.54 μm as a function of the reverse bias voltage (and for a constant current through the device) reveal that excitation of Er only occurs at voltages above 1 V, demonstrating that impact is the pumping mechanism. Moreover, we have found that Er ions are only excited within ∼15 nm from the edges of the depletion layer leaving a dark, ∼50 nm thick, region in the central part of the depletion region. Monte Carlo calculations confirmed that only close to the depletion layer edges the energy gained by the carriers in the electric field is high enough to impact excite Er.