Steady-state and time-resolved spectroscopy of porous silicon

We present experimental data on steady-state properties, time-resolved properties and on polarization characteristics of porous silicon photoluminescence and models for the decay processes of the red-orange band. The manifold manifestation of inhomogeneous broadening of this band in emission, excitation, polarization, kinetics and degradation supports the model in which porous silicon is treated as a network of crystallites connected via an oxide interface. Spectral inhomogeneties of the red-orange band can be described in terms of varying shape and size of silicon clusters. The polarization of emission is explained by coexistence of dot-like and wire-like entities, i.e. spherical and non-spherical clusters. The relative weight of these species determines the polarization degree, whereas the kinetics are controlled by the transport of excitations among the clusters. The decay is modeled by a modified stretched exponential function with the local lifetime, the migration lifetime, and a scaling factor. The latter is determined by the dimensionality of the space available for migration which was found to be close to but less than unity. On the nanosecond range two distinct bands in the blue-green region are evaluated that need further studies for interpretation. Generally, arguments are proposed in favor of a quantum confinement origin of the red-orange band and a bridge between quantum-wire and quantum-dot models is provided.     

Investigation of mechanical strains in porous silicon by difference Raman spectroscopy

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 49, No. 4, pp. 612–615, October, 1988.     

Acoustic phenomena in filled porous glasses by time-resolved spectroscopy

We studied the propagation of ultrasonic acoustic waves in two porous glasses characterized by different pore diameters (4 and 200 nm) filled with carbon tetrachloride. Using a time-resolved optical spectroscopic technique (Transient Grating) we can measure the acoustic dynamics of these systems. The sound velocities, measured in both the samples, show high values compared with the sound velocity of bulk carbon tetrachloride; they cannot be predicted by the simple effective medium model, but are in good agreement with the estimate obtained from the Biot theory.     

Efficient luminescence from porous silicon

Photoluminescence measurements are carried out on porous silicon layers. We show the enhancement and stabilization of the luminescence when depositing a silicon nitride layer on top of porous layers.We also demonstrate that direct- and remote-plasma nitridation are good ways to reduce the ageing effect of porous silicon layers due to a passivation of dangling bonds.     

Irreversible quenching of luminescence in porous silicon

The influence of applied voltage on photoluminescence (PL) in porous silicon was studied. A strong PL band around 680 nm was observed when excited by a 300 nm ultraviolet light with no voltage applied, but upon increasing the bias voltage, a strong and progressive decrease of the PL intensity was observed leading finally to a complete quenching of the emitted light at 1.80 V. The peak position of the emission appears to be stable. This effect is completely irreversible, and the spectra depend on the increased voltage to the sample and corresponding temperature increase. Nonradiative recombination resulting from the thermal oxidation was suggested to be responsible for the quenching.     

Temperature effect on porous silicon luminescence

A theoretical surface-state model of porousilicon luminescence is proposed. The temperature effect on the PhotoLuminescence (PL) spectrum for pillar and spherical structures is considered, and it is found that the effect is dependent on the doping concentration, the excitation strength, and the shape and dimensions of the Si microstructure. The doping concentration has an effect on the PL intensity at high temperatures and the excitation strength has an effect on the PL intensity at low temperatures. The variations of the PL intensity with temperature are different for the pillar and spherical structures. At low temperatures the PL intensity increases in the pillar structure, while in the spherical structure the PL intensity decreases as the temperature increases, at high temperatures the PL intensities have a maximum for both models. The temperature, at which the PL intensity reaches its maximum, depends on the doping concentration. The PL spectrum has a broader peak structure in the spherical structure than in the pillar structure. The theoretical results are in agreement with experimental results.     

Properties of liquid silicon observed by time-resolved x-ray absorption spectroscopy

Time-resolved x-ray spectroscopy at the Si L edges is used to probe the electronic structure of an amorphous Si foil as it melts following absorption of an ultrafast laser pulse. Picosecond temporal resolution allows observation of the transient liquid phase before vaporization and before the liquid breaks up into droplets. The melting causes changes in the spectrum that match predictions of molecular dynamics and ab initio x-ray absorption codes.     

Evidence of localized luminescence centers in porous silicon

The mechanism by which porous silicon luminesces when excited by UV radiation is still unknown. As opposed to de-localized models, proposed by other researchers, in which conduction band carriers are believed to recombine with traps during luminescence, we show evidence for and argue ha favour of a localized structure in which, on excitation, electrons are promoted to higher energy levels within a luminescence center but are not given up to the conduction band. Luminescence occurs when they relax to a lower level in the center.     

Strongly nonlinear luminescence in oxidized porous silicon films

Partially oxidized free-standing porous silicon films show a strongly superlinear increase in photoluminescence (PL) intensity above a threshold cw excitation intensity of 10 W/cm². The PL-intensity increase can be expressed by a power law with n9 as a function of the excitation intensity. The PL-peak wavelength of this emission is slightly redshifted from that at low-excitation levels. These changes are fully reversible and reproducible, but not observed in samples on substrate. We attribute this behavior to thermal reexcitation of carriers trapped at the dangling bond states in initially nonluminescent Si nanocrystallites.     

多孔硅结构与发光机理的非线性光学研究

最近的研究中,采用1.06μm超短脉冲光激发,在多孔硅表面观察到了有效的红外多光子激发的荧光发射.研究表明,这是一个增强的三阶非线性光学过程.本文利用其三阶非线性特性对具有强可见光发射的多孔硅结构进行了研究,结果显示晶体硅的各向异性特征在多孔硅中几乎被保留;此外,较强的激光激发导致的红外上转换荧光信号衰减过程被归结为与多孔硅表面氢的脱附有关.     

Study of birefringence in porous silicon layers by IR Fourier spectroscopy

Birefringence in porous silicon layers prepared with different etching currents on a (110) substrate is studied by IR Fourier spectroscopy. The spectra exhibit beats in the intensity of transmitted and reflected radiation due to the summation of the intensities of the ordinary and extraordinary waves interfering in the porous layer. An analysis of the spectra shows the layers to exhibit properties of a negative uniaxial crystal with the optical axis lying in the layer plane. The difference between the refractive indices of the ordinary and extraordinary waves for a layer with a porosity of 80% reaches 18%. The experimental data are in agreement with the calculations based on the effective-medium approximation, which takes into account the anisotropy of silicon nanocrystal arrangement in a porous layer.     

X-ray luminescence of disperse SiO2 and porous silicon

We carry out a comparison between the luminescence spectra (photo-and x-ray luminescence) of porous silicon and disperse SiO₂, which by its physical characteristics is most similar to oxide films on porous silicon. The photoluminescence of porous silicon was also investigated using fluorescence (excitation by a nitrogen laser) and metallographic microscopes. We found that the natures of the luminescence centers of porous silicon and disperse SiO₂ are identical. A porous layer on single-crystal silicon ensures the creation of a highly branched surface of oxide film. Luminescence centers are located on its inner (as viewed from the porous silicon) surface. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 247–251, March–April, 1998.     

Investigation of the mechanisms of luminescence of ZnS−Ag(Al) single crystals by the method of time-resolved spectroscopy

By the methods of amplitude and time-resolved spectroscopies the mechanisms of luminescence of ZnS−Ag(Al) single crystals differing in dislocation density are studied in pulse laser excitation. Experimental results are analyzed based on the theory of donor—acceptor pairs and the theory of centers surrounded with defect clouds. The data obtained are in agreement with the second theory. Ural State Technical University, 19 Mir Str., Ekaterinburg, 620002, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 857–860, November–December, 1999.