K. Luterová

Optical gain in porous silicon grains embedded in sol-gel derived SiO2 matrix under femtosecond excitation

Porous silicon grains embedded in the phosphorus doped SiO2 matrix exhibit improved photoluminesce properties and better stability in comparison with native porous silicon samples. We have tested this material for the presence of room temperature optical amplification under femtosecond (100 fs, 395 nm) excitation. Combined variable stripe length and shifted excitation spot experiments reveal positive optical gain, the net modal gain coefficient reaching 25 cm−1 at a pump intensity of 1.1 W/cm2 (mean power). The gain spectrum is broad (full width at half maximum ∼130 nm), peaked at ∼650 nm, and is slightly blueshifted with regard to the standard photoluminescence emission.

Stimulated emission in blue-emitting Si+-implanted SiO2 films?

We investigate the blue photoluminescence of Si+-implanted SiO2 films under picosecond UV excitation. The emission intensity exhibits a nonlinear increase with increasing excitation intensities, accompanied by pulse shortening. The photoluminescence decays nonmonoexponentially in time. However, the nonlinearities are not associated with significant spectral narrowing. To explain the results, we propose and numerically investigate a kinetic model based on competition between radiative (both spontaneous and stimulated) and nonradiative recombination in isolated luminescence centers in the SiO2 matrix. Good agreement between theoretical and experimental data seems to confirm the existence of stimulated emission in the films, however, under extremely high excitation densities only (approximately 100 MW/cm2).

Active planar optical waveguide made from luminescent silicon nanocrystals

We show experimentally that a layer of silicon nanocrystals, prepared by the Si-ion implantation (with the energy of 400 keV) into a synthetic silica slab and exhibiting room-temperature red photol ...

Red electroluminescence in Si+-implanted sol–gel-derived SiO2 films

We report on a continuously emitting electroluminescent device fabricated by Si+-ion implantation and subsequent annealing of a SiO2 layer on a silicon substrate. The SiO2 layer with a thickness of 250 nm was prepared by the sol–gel technique. Four different Si+-ion energies and implantation doses were applied in order to obtain a flat Si+-ion profile across the SiO2 film thickness with an atomic Si excess of 5%. Electroluminescence (EL) occurs above a low-voltage threshold (∼5 V, 1 A/cm2) at one bias polarity only even if the device in fact does not exhibit rectifying properties. EL microscopy reveals that EL at 295 K is emitted from a small number of bright spots with diffraction-limited size. EL spectra of individual bright spots were measured using an imaging spectrometer. The wide EL emission band (situated in the red region ∼750 nm) obtained with spatial averaging over the semitransparent indium–tin–oxide contact represents the envelope of these individual contributions. We suggest that the EL is du...

Instabilities in electroluminescent porous silicon diodes

The stability of high quantum efficiency, porous silicon (PS) electroluminescent (EL) diodes have been analyzed under pulsed operation. The results show that EL exhausting is due to a charging of the PS network which recovery is thermally activated (Er∼0.4 eV). This suggests a charge trapping mechanism at larger crystallites with low bandgap, inhibiting further injection of carriers. Also, forward current is found to be thermally activated, however, with a distinctly lower activation energy (Ej∼0.1–0.2 eV), possibly related to jumping of carriers over barriers between nearby crystallites. Finally, high current densities may lead to thermal runaway causing permanent damage to the structure.

VISIBLE PHOTOLUMINESCENCE IN HYDROGENATED AMORPHOUS SILICON GROWN IN MICROWAVE PLASMA FROM SIH4 STRONGLY DILUTED WITH HE

Room temperature visible photoluminescence (PL) of wide-band gap hydrogenated amorphous silicon (a-Si:H) thin films prepared in SiH4 microwave plasma strongly diluted with He is reported. Films were characterized by means of optical and infrared absorption, hydrogen thermal desorption, and Raman scattering. The band gap of a-Si:H films varies within the interval 2.0–2.2 eV, corresponding PL maxima are located at 1.4–1.6 eV. The highest PL intensity was observed in samples with a position of H–Si–H symmetric stretching vibration of the –(SiH2)n– units near the frequency of 2100 cm−1. The strong evidence for two distinct types of PL processes is presented: one being linked with oligosilanes and the second one attributed to electron-hole recombination in tail states.

Optical Gain Measurements With Variable Stripe Length Technique

Optical gain in semiconductors plays a decisive role when evaluating potential of a given semiconducting material in photonic applications: The occurrence of positive optical gain is a necessary (albeit insufficient) condition for realisation of a semiconductor laser diode. It is therefore highly desirable to have a simple and reliable experimental method enabling us to get information about the presence/absence of gain and, in affirmative case, about its magnitude. Despite the fact that semiconductor lasers are pumped by electric current, the method must be purely optical so as to circumvent possible complications arising from the necessity of fabrication of suitable electric contacts and p-n junctions, detrimental Joule heating etc. In other words, the method should be based on optical rather than electrical pumping.

Silicon thin films deposited at very low substrate temperatures

Abstract A series of investigations were performed to study the influence of the substrate temperature on the structure and properties of silicon thin films. Substrate temperature was varied in the wide range of 35–200 °C. It has been shown that the films grown below 60 °C exhibit an unusual structural behavior. A sharp TO phonon peak at 520 cm −1 was detected in Raman spectra, which is associated with the crystalline structure. In contrast to these results, the same samples do not show any crystallite-related peak by X-ray diffraction and their optoelectronic properties (dark conductivity, activation energy and subgap absorption spectra) show amorphous features. A similar discrepancy was observed for a hydrogen dilution ratio ( r H =([SiH 4 ]+[H 2 ])/[SiH 4 ]) series of samples deposited at 60 °C. Hydrogen dilution ratio was varied from 25 to 170. It seems that at low substrate temperature a parameter window exists where the silicon thin films can be grown with the properties combining both crystalline and amorphous behavior.

Room temperature electric field induced crystallization of wide band gap hydrogenated amorphous silicon

Abstract Our results indicate that macroscopic areas (∼mm 2 ) of thin films of hydrogenated amorphous Si (a-Si:H) with high hydrogen content (20–45 at.% H) can be crystallized very simply by applying an above-threshold (∼2×10 5 V/cm) an electric field perpendicular to the film. We investigate p + –i–n + and p + –p–n–n + a-Si:H structures sandwiched between CrNi and ITO electric contacts. The crystallization is achieved at room temperature and normal air pressure. We believe this is a new type of metal-induced crystallization in solid phase, mediated by Ni (or Cr) silicides that are formed from the CrNi contacts. High concentration of voids in our high-hydrogen content a-Si:H structures may contribute to easy migration of the silicides, thereby catalyzing the easy crystallization.

Active planar optical waveguides with silicon nanocrystals : Leaky modes under different ambient conditions

We study both experimentally and theoretically the propagation of light emitted from silicon nanocrystals forming planar waveguides buried in SiO2. Photoluminescence spectra detected from the sample facet show significant spectral narrowing—leaky modes—with respect to the spectra measured in standard photoluminescence configuration. The spectral position of the leaky modes responds strongly to a local change of refractive index (liquid drop) on the sample surface. Higher refractive index of the liquid induces higher redshift of the mode position. Experimental data agree with the previously proposed leaky mode model.