C. Spinella

Correlation between luminescence and structural properties of Si nanocrystals

Strong room-temperature photoluminescence (PL) in the wavelength range 650–950 nm has been observed in high temperature annealed (1000–1300 °C) substoichiometric silicon oxide (SiOx) thin films prepared by plasma enhanced chemical vapor deposition. A marked redshift of the luminescence peak has been detected by increasing the Si concentration of the SiOx films, as well as the annealing temperature. The integrated intensity of the PL peaks spans along two orders of magnitude, and, as a general trend, increases with the annealing temperature up to 1250 °C. Transmission electron microscopy analyses have demonstrated that Si nanocrystals (nc), having a mean radius ranging between 0.7 and 2.1 nm, are present in the annealed samples. Each sample is characterized by a peculiar Si nc size distribution that can be fitted with a Gaussian curve; by increasing the Si content and/or the annealing temperature of the SiOx samples, the distributions become wider and their mean value increases. The strong correlation betw...

Formation and evolution of luminescent Si nanoclusters produced by thermal annealing of SiOx films

Si nanoclusters embedded in SiO2 have been produced by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition. The structural properties of the system have been investigated by energy filtered transmission electron microscopy (EFTEM). EFTEM has evidenced the presence of a relevant contribution of amorphous nanostructures, not detectable by using the more conventional dark field transmission electron microscopy technique. By also taking into account this contribution, an accurate quantitative description of the evolution of the samples upon thermal annealing has been accomplished. In particular, the temperatures at which the nucleation of amorphous and crystalline Si nanoclusters starts have been determined. Furthermore, the nanocluster mean radius and density have been determined as a function of the annealing temperature. Finally, the optical and the structural properties of the system have been compared, to demonstrate that the photoluminescence properties of the system de...

Quantum confinement and recombination dynamics in silicon nanocrystals embedded in Si/SiO2 superlattices

In this study the structural and optical properties of nanocrystalline Si/SiO2 superlattices have been investigated and discussed. Ordered planar arrays of silicon nanocrystals (Si-nc) have been formed by thermal annealing of ten period amorphous Si/SiO2 superlattices prepared by plasma enhanced chemical vapor deposition. Thermal processing of the superlattices results in well separated (by about 5 nm of SiO2) nanocrystalline Si layers, when the annealing temperature does not exceed 1200 °C. The photoluminescence (PL) properties of these layers have been studied in details. The PL peaks wavelength has been found to depend on the laser pump power; this intriguing dependence, consisting in a marked blueshift for increasing power, has been explained in terms of the longer lifetime characterizing larger Si-nc. It is also observed that these decay lifetimes exhibit a single exponential behavior over more than two orders of magnitude, in clear contrast with the typical, nonsingle exponential trends observed for...

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 ...

Electrical and optical characterization of Er‐implanted Si: The role of impurities and defects

The electrical and optical properties of Er‐implanted Si are shown to be critically dependent on the presence of impurities and defects. A large enhancement in the electrical activation of Er (up to three orders of magnitude) is obtained by coimplanting Er with O or C at 300 °C. The use of C also allows one to obtain a good quality crystal after implantation and annealing. This is shown to be crucial in the photoluminescence process. In fact, in spite of the large amount of active Er atoms, photoluminescence is inhibited in the presence of the high concentration of precipitates and crystallographic defects which are left after annealing of the Er and O coimplants. The photoluminescence intensity is, on the other hand, enhanced by the high concentration of active Er atoms in the defect‐free crystal which is left after annealing of the Er and C coimplants. Moreover, a clear shift in the main photoluminescence peaks is observed in Er‐ and C‐coimplanted samples as a result of the different surroundings experi...

Degradation and hard breakdown transient of thin gate oxides in metal–SiO2–Si capacitors: Dependence on oxide thickness

We have investigated the dynamics of hard intrinsic dielectric breakdown of gate oxide layers with thickness between 35 and 5.6 nm in n+ polycrystalline Si–SiO2–Si metal/oxide/semiconductor capacitors after constant voltage Fowler–Nordheim stress. The buildup of defects in the oxide during the degradation phase was monitored by quasi static C–V measurements. The dynamics of the final breakdown event was followed with high time resolution, allowing to measure voltage, current, and power versus time during the breakdown transient. Transmission electron microscopy data quantifying the damage produced during this transient are reported. Finally, we propose a phenomenological model concerning the dynamics of breakdown with model parameters adjusted on the basis of the experimental data.

Fluorine effect on As diffusion in Ge

The enhanced diffusion of donor atoms, via a vacancy (V)-mechanism, severely affects the realization of ultrahigh doped regions in miniaturized germanium (Ge) based devices. In this work, we report a study about the effect of fluorine (F) on the diffusion of arsenic (As) in Ge and give insights on the physical mechanisms involved. With these aims we employed experiments in Ge co-implanted with F and As and density functional theory calculations. We demonstrate that the implantation of F enriches the Ge matrix in V, causing an enhanced diffusion of As within the layer amorphized by F and As implantation and subsequently regrown by solid phase epitaxy. Next to the end-of-range damaged region F forms complexes with Ge interstitials, that act as sinks for V and induce an abrupt suppression of As diffusion. The interaction of Ge interstitials with fluorine interstitials is confirmed by theoretical calculations. Finally, we prove that a possible F-As chemical interaction does not play any significant role on do...

Origin and perspectives of the 1.54 μm luminescence from ion-beam-synthesized β-FeSi2 precipitates in Si

The structural and optical properties of β-FeSi2 precipitates in Si have been analyzed. Float zone Si samples were implanted at 250 °C with 350 keV Fe ions to fluences in the range 1–5×1015/cm2 and annealed in vacuum at 800 °C for times up to 24 h. Detailed morphological analyses of these samples, using transmission electron microscopy, reveal the presence of (i) a band of small (with a diameter 100 nm diameter) fully relaxed β-FeSi2 precipitates centered at a depth of ∼320 nm, and (iii) residual extended defects. A sharp photoluminescence peak at 1.54 μm is measured at 17 K. This peak remains unchanged when the region containing the small precipitates is removed, using Ar sputtering. On the other hand, it is also fully suppressed when the large precipitates region is removed and a high concentration of extended defects remains in the samples. This allowed us to identify the large unstrained precipitates as...

Transition from small interstitial clusters to extended {311} defects in ion-implanted Si

We have investigated the transition from small interstitial clusters to {311} defects in ion-implanted Si. Czochralski Si wafers were implanted with 1.2 MeV Si ions to fluences in the range 1012–5×1013/cm2 and annealed at temperatures of 600–750 °C for times as long as 15 h. Photoluminescence and transmission electron microscopy analyses allowed us to analyze the transition of small interstitial clusters, formed by the agglomeration of the excess interstitials introduced by the beam, into {311} defects. It is found that {311} defects form only at fluences ⩾1013/cm2 and at temperatures above 600 °C. When {311} are observed in transmission electron microscopy, the luminescence spectrum is dominated by a sharp signal at 1376 nm which has been correlated with optical transitions occurring at or close to these defects. At lower temperatures or at lower fluence, no extended defects are observed in transmission electron microscopy and the luminescence spectrum present two broad signatures arising from carrier re...

Electrical and thermal transient during dielectric breakdown of thin oxides in metal-SiO2-silicon capacitors

The dielectric breakdown of gate oxide layers with thickness of 35 and 9.3 nm in metal-oxide-semiconductor capacitors with a n+ polycrystalline Si/SiO2/n− Si stack was investigated. Breakdown was characterized in a particular circuit configuration by following the time evolution of voltage, current, and power through the capacitor with a time resolution of the order of 2 ns. A detailed morphological characterization of the damaged samples by emission and transmission electron microscopy is shown and discussed. The results of the morphological analysis and of the electrical measurements are quantitatively discussed by simulating, through heat-flow calculations, the time evolution of the temperature in the regions interested to the breakdown phenomenon.