The effect of monochromatic light during the anodization of porous silicon is investigated from the points of view of structural characterization and photoluminescence. A clear correlation is observed between the photoluminescence peak energy and the bandgap energy determined by the illuminating photon energy. The difference of these two energies is analyzed by a size-dependent Stokes shift.

Polarization memory effect in the porous Si photoluminescence is studied. The anisotropy of the linear polarization degree is found in the samples etched with polarized light-assistance. The effect is explained by the anisotropie in plane distribution of the elongated Si crystallites. Under resonant optical excitation four-fold anisotropy of the photoluminescence polarization, linked to the crystalline axes of the bulk Si substrate, is observed.

Photoluminescence from porous silicon was examined using synchrotron radiation as an excitation source. The tunability of the excitation source permitted a wide range of excitation energies ranging from VUV to X-rays. This permitted site selective excitation where specific core levels (i.e. Si-K, O-K, Si-2p) were probed. In high porosity samples, luminescence bands of both surface and bulk origins were observed. All experiments exhibited a common luminescence maximum typically in the orange-red region of the visible spectrum. At certain specific excitation energies particularly in the VUV region additional peaks related to sites with oxygen character were also observed. The VUV excitation luminescence spectra of the porous silicon remarkably resembled that of oxygen deficient amorphous silicon dioxide glasses.

A pulsed, high-power TEA CO2 laser with Unes in the region from 9.2 to 10.6 μm has been used to irradiate luminescent porous Si samples. The IR laser pulses heat the sample on a time scale much shorter than the PL decay time which is at 300 K for the PL at 1.65 eV in the order of tenth of μs. One IR pulse serves to increase the temperature of the luminescing particles in ∼2 μs up to 100°C. This increase of temperature leads to a efficient reduction of the photoluminescence (PL) intensity. However, the PL decay times are almost not affected by the heating pulse. Based on this measurement a picture of the recombination statistics that takes account of the granular Nature of the material is developed.

Photoluminescence saturation under intense CW optical excitation and optical degradation of photoluminescence from porous Si are studied. The anisotropy of the luminescence is observed under intense linearly polarized illumination at room temperature and after polarized light induced degradation at low temperature. The Auger process is shown to be responsible for these observations.

Highly luminescent free-standing porous silicon thin films of excellent optical quality have been manufactured by using electrochemical etching and lift-off steps combined with supercritical drying. One to 50 μm thick free-standing layers made from highly (p+) and moderately (p) Boron doped single crystal silicon (c-Si) substrates have been produced with porosities (P) up to 95 %. The Fabry-Pérot fringes observed in the transmission and photoluminescence (PL) spectra are used to determine the refractive index. At the highest P the index of refraction is below 1.2 from the IR to 2 eV. The absorption coefficients follow a nearly exponential behavior in the energy range from 1.2 eV and 4 eV. The porosity corrected absorption spectra of free-standing films made from p type c-Si substrates are blue shifted with respect to those prepared from p+ substrates. For P > 70 % a blue shift is also observed in PL. At equal porosities the luminescence intensities of porous silicon films made from p+ and p type c-Si are different by one order of magnitude.

We have measured the Raman spectrum of porous silicon layers for different light polarizations. For the polarizations where the first-order Raman peak is forbidden in bulk silicon, porous silicon shows a band. This band is almost as intense as the one in the allowed polarizations but its lineshape is very different. The ‘forbidden’ band is usually wider and shifted to lower energies with respect to the allowed band. Both bands are analyzed in terms of sample characteristics, layer depth, and excitation wavelength.

The response of a range of porous Si and poly Si films to storage in acellular simulated body fluids is summarised and its implications discussed. It is suggested that the combination of VLSI technology, micromachining and surface microstructuring achievable with silicon, could establish this prominent semiconductor as a very useful biomaterial by the next century. The ‘biocompatibility’ of a variety of silicon microstructures, and even bulk silicon has received surprisingly little study, but now warrants detailed in-vitro and in-vivo assessment.

This paper describes the fabrication method and initial characterization of self-assembled mesoscale arrays of quantum-confined CdS nanoparticles using DNA as a template for the overall shape. Three DNAs were used: the circular and linear forms of the plasmid pUCLeu4, and circular φ×174 RF II. In all three cases, the mesoscale lengths are consistent with the A-form of DNA. The structural signatures and crystallography were confirmed using conventional and high resolution transmission electron microscopy, and electron diffraction. Optical spectroscopy demonstrated that the particles display quantum-confinement effects. This research is a fundamental demonstration of the power of combining biochemical and solid-state processing techniques.

Semiconductor quantum dots of cadmium sulfide, CdS, are approximately the size of proteins and are photoluminescent in the red, yellow, or green, depending on surface preparation. This photoluminescence is very sensitive to the Nature and amount of adsorbates. We have found that DNAs with intrinsic curvature adsorb more strongly to the surface of 47 Å CdS quantum dots, as judged by concentration-dependent changes in photoluminescence. The binding constants we obtain are similar to those found for nonspecific protein-DNA interactions. The surface groups of the CdS substrate also influence DNA adsorption. Thus these protein-sized colloidal particles can be used in chemical sensing applications for curved or kinked DNA; DNA with unusual structures is thought to influence biological function such as transcription.

Light induced electron transfer (ET) from nanosize semiconductors of MoS2 to organic electron acceptors such as 2,2′-bipyridine (bpy) and methyl substituted 4,4′,5,5′-tetramethyl-2,2′-bipyridine (tmb) was studied by static and time resolved photoluminescence spectroscopy. The kinetics of ET were varied by changing the nanocluster size (the band gap), the electron acceptor, and the polarity of the solvent. MoS2is an especially interesting semiconductor material as it is an indirect semiconductor in bulk form, and has a layered covalent bonding arrangement which is highly resistant to photocorrosion.

In order to investigate the fundamentals of electron migration in nanostructured metal-oxide semiconductors, the transient photocurrent response of dye-sensitized porous nanocrystalline TiO2 is studied. The time-resolved photocurrent response at light steps or pulses shows a faster transient upon increasing the light intensity. Intensity-modulated photocurrent spectroscopy (IMPS) reveals that the transient photocurrent is dominated by two time constants, i.e. the geometrical one and a characteristic time related to electron trapping. A theoretical model is derived in which the occupation dynamics of a single electron trap is considered using Shockley-Read-Hall kinetics. The geometrical RC time of the electrode is also included. Excellent agreement between this model and the measured IMPS spectra is obtained.

Transient photocurrent measurements are reported in an electroluminescent porous silicon diode. Electron drift mobilities are obtained from the data as a function of temperature. Electron transport is dispersive, with a typical dispersion parameter α≈ 0.5. The range of mobilities is 10−5 − 10−4 cm2Vs between 225 K amd 400 K. This temperature-dependence is much less than expected for multiple-trapping models for dispersion, and suggests that a fractal structure causes the dispersion and the small mobilities.

We present the temperature and frequency dependence of the current-voltage (I-V) characteristics of Al barriers deposited on porous Si grown on p-type Si substrates. These barriers exhibit a rectifying behaviour when the temperature is higher than 250 K. The I-V characteristics can be understood by a conduction in porous Si taking place via free electrons thermally excited from Pb centers associated with the existence of a SiO2-Si interface and via hopping between these Pb centers.

We report the fabrication of photovoltaic devices by the anodization of microcrystalline silicon films on single crystal silicon substrates. The porosity of the films was varied from 20% to 60% by changing the anodization conditions. An unetched μc-Si based device was used for reference. The influence of the porosity on the series resistance (Rs), the reflectance, and the spectral response of the devices was studied in detail. In order to determine Rs, the current-voltage characteristics were analyzed, both in the dark and under illumination. We observed that the value of Rs increased from 3.1 Ω to 97 Ω and the value of the reflectance decreased from 24% to 7% when the porosity increased from 20% to 60%. Initially, an optimum device performance (fill factor of 0.53 and efficiency of 7.2%) was achieved for a porosity of 40%, which was about a 40% improvement as compared to the reference (unetched) μc-si based device. Due to a further reduction in Rs by using an intermediate ITO layer and a superior grid-contact architecture, a device efficiency of 10% has been recently achieved.

We have produced lateral porosity modulation in porous silicon layers on a micron scale. Using the photosensitivity of the etching process and optical interferences, stripes are formed periodically on the top but also in the depth of the layer depending on the illumination wavelength. The periodicity of the structure is easily modified by changing the wavelength or the incident angles of the two laser beams used to create the modulated illumination. By rotating the sample, two dimensional structures have also been obtained. The samples formed by this procedure are characterised by light diffraction and photoluminescence both of which show the alternance of high and low porosity ranges. With this kind of in-depth lithography, easy and very cheap fabrication of porous silicon gratings promises a large potential range of applications of this material to integrated optics and photonics.

Porous silicon multilayers are easily produced by a variation of the current density during the electrochemical etching process. The obtained porosity profile in depth is equivalent to a refractive index profile. Preparing appropriate multilayer systems one can design optical filters with sharp reflectivity peaks from the infrared through the whole visible spectral range (dielectric mirrors). Stability in time is achieved applying a suitable oxidation process. In many cases of optical process and material control the required spectral decomposition of the probing light can be done with sufficient accuracy by a set of porous silicon optical filters whose spectral characteristics can lbe adapted easily to the given problem. In this work we present the prototype of an optical sensor that is a low-cost alternative to ‘real’ (and expensive) spectrometers. Application examples in the infrared and visible are given to demonstrate the sensor performance;.

Recently passive optical devices like filter structures or waveguides based on porous silicon have attracted high interest due to their easy and cheap fabrication. We have formed interference filters using porous silicon by changing the current density during formation. For the specific design of these filter structures a calibration of the etch rate and the refractive indices is required. Therefore we have determined the effective dielectric function for different substrate doping levels and anodization current densities by fitting reflectance spectra. Based on these results different kinds of reflectance filters consisting of discrete layers (Bragg reflectors, Fabry-Perot filters) as well as filters with a continuous change of the refractive indices with depth (rugate-filters) can be realised. Furthermore we present applications of these filter structures such as anti-reflectance coatings and high quality mirrors.

Using STM assisted chemical vapor deposition (STM-CVD), we have successfully fabricated what we believe are the finest gratings to date. Our best grating consists of half-micron long evenly spaced lines which have a width of 10 nm and a 10 nm interspacing. The gratings were fabricated on passivated p-type silicon <111> using trimethylaluminum (TMA) as a precursor gas. Although the composition of the nanostructures is not exactly known at this time, we believe that the lines are partially metallic. We made some systematic investigations to optimize the resolution, including studies of the effect of tunneling current, tip speed, tip-sample biasing voltage, and the effect of formation of very fine TMA whiskers on the tip.

Thermal oxide layers on Si which have been implanted with Si conduct electrical current and emit light. The electroluminescence effect (EL) has an efficiency which is comparable to the best values that have been reported for the porous Si based devices. Generally, the EL spectrum differs substantially from the photoexcited luminescence. It is linked with that excited by external high energy electrons in cathodoluminescence (CL) experiments. We suggest to consider the effect as internal CL. Based on studies of transport and EL characteristic we evaluate the possibilities of an electroluminescent device based on such layers.