Porous Silicon Interface Research Articles

STRUCTURAL AND OPTICAL CHARACTERIZATION OF ITO/PS HETEROJUNCTION

Transparent conducting indium tin oxide (ITO) films are deposited on Porous Silicon (PS) substrates by spray pyrolysis technique. In this process, the films are formed over the surface and also incorporated into the pores of PS and thereby making a protecting layer as well as a contacting terminal. Thus, the ITO/PS/ Si heterojunction light-emitting devices are fabricated. The growth of ITO on PS is thoroughly investigated by SEM and X-ray diffraction techniques. The features of growth on other substrates like single-crystal p-type (100) silicon and glass are also taken into consideration. The influence on the PS interface is correlated with the electrical and luminescent behavior of the resulting heterojunction diode structure.

Lattice distortion in nanostructured porous silicon

High-resolution transmission electron microscopy in combination with image analysis was used to perform a detailed analysis of the lattice structure of the Si nanocrystals that compose porous silicon (PS) as a function of depth. It has been observed that the morphology, crystallinity, and orientation of porous silicon change strikingly from that of Si depending on depth. In addition to misalignment produced by the formation process, a nonsymmetrical distortion of the Si cell away from cubic has been found. Thus, the resulting lattice structure of porous silicon could be triclinic and partially nonperiodic. These effects become more prominent with distance away from the Si∕PS interface.

Optical properties of porous silicon. Part III: Comparison of experimental and theoretical results

Abstract In our previous study, the refractive indices of freestanding porous silicon (PS) layers were derived using the envelope method, where the computation is based on the values of local minima and maxima in the oscillations of transmission spectra. In the present work, an improved procedure for calculating the optical parameters from the measurements data is described. It is verified by reflection measurements on freestanding samples that optical scattering at the air–PS interface is the main reason for the loss of the transmitted light intensity and thus for the inaccurate results we obtained earlier by the envelope method. This however can be avoided by taking into consideration the relationship between the optical path in the plane-parallel film and the position of extrema in the transmission spectra. The as-determined effective refractive indices show very good matching with the theoretical calculations by the Bruggeman’s effective medium approximation.

Humidity-voltaic characteristics of Au–porous silicon interfaces

The humidity-voltaic effect, i.e. generation of open-circuit voltage (Voc), in Au–porous silicon (PS) interface in humid atmosphere (up to 450 mV at 95% relative humidity) in dark and daylight illumination is discovered. The humidity-stimulated voltage generation is attributed to the splitting of water and hydrogen molecules on the surfaces of the Au catalyst, where further diffusion penetration of hydrogen ions into the Au–PS interface results in the formation of dipoles, thus inducing Voc across dipoles. The generation of Voc (up to 550 mV) has also been observed on dipping of Au–PS structures in different hydrogen-containing solutions (ethanol, benzine, sodium tetraborate pentahydrate, etc). Data of response time-dependent changes of the open-circuit voltage generated in Au–PS structures under humid conditions were used for the estimation of diffusion coefficients of hydrogen. The temperature dependence of the diffusion coefficient of hydrogen at 323–353 K via the pore surfaces of PS is attributed to be D = 1.3 × 10−2exp (−0.25/kT).

Electrically induced luminescence quenching in p+-type and anodically oxidized n-type wet porous silicon

Electrically induced photoluminescence (PL) quenching from reverse-biased p+-type porous silicon (PS) and forward biased oxidized n-type PS, as well as their dynamics, have been investigated. The two systems exhibit common properties. The energy selectivity of the PL quenching by the applied potential is much lower than with nonoxidized n-type PS. The PL quenching time is independent of the emission energy. It is governed by carrier injection into PS and is limited by the Helmoltz capacitance at the electrolyte–PS interface and the electrolyte resistivity. The PL recovery time is longer than the quenching time and increases as the emission energy decreases. It is limited by the transparency of the energy barrier (due to oxide in the case of oxidized n-type PS and the depletion region in the case of p+-type PS) at the PS–substrate interface. It is shown that electron storage into PS is possible for time in excess of several seconds depending on parameters such as the oxidation level, the PS thickness, and the electrolyte resistivity.

Small-angle X-ray scattering studies on oxide layer thickness at the porous silicon interface

We have determined the thickness of an oxide layer at the p+-type porous silicon interface as a function of oxidation time, by using a small angle X-ray scattering (SAXS). The scattering experiments were carried out using a Kratky camera with a step-scanning device. Oxidation was achieved by storing the porous silicon samples in various temperatures under high relative humidity. The negative deviations from Porod's law were observed from the scattering curves of oxidized samples. The oxide layer thickness was determined from the scattering curve using a sigmoidal-gradient approximation for the diffuse boundary. The oxide layer thickness values as a function of oxidation time, obtained using SAXS are compared to measured weight increase values, caused by the oxidation.

Hydrogen sensing characteristics of Cu–PS–Si structures

The effect of ambient humidity on the current–voltage characteristics of Cu–PS (porous silicon) structures was investigated. The humidity-voltaic effect, i.e. generation of humid voltage (Voc) in Cu–PS interface in humid atmosphere (up to 300 mV at 95% relative humidity) in dark and day-light illumination is discovered. Humidity-stimulated voltage generation is caused by the hydrogen component of water vapour of ambient. A possible mechanism of hydrogen-stimulated voltage generation in Cu–PS interface is suggested. Besides the effect of annealing in the range of 60–200°C in air on Voc of Cu–PS structures was studied and decrease of values of Voc depending on duration of annealing was discovered. These changes were attributed to diffusion of oxygen from air and oxidation of the copper film. (Voc−t) data were used for estimation of the diffusion coefficients of oxygen in the Cu film. The temperature dependence of the oxygen diffusion coefficient in Cu films is described by the relation D = 5.2×10−7exp(−0.44/kT).

Microstructure of indium tin oxide films deposited on porous silicon by rf-sputtering

Transparent conducting indium tin oxide (ITO) films are deposited by rf-sputtering at a constant power of 400 W in Ar atmosphere on the porous silicon (PS) layers anodized on p-type (100) Si wafers. At three successive stages of deposition for 10, 20 and 30 min, respectively, the growth of ITO on PS is thoroughly investigated by atomic force microscopic (AFM), SEM and X-ray diffraction (XRD) techniques. The features of growth on other substrates like single crystal p-type (100) silicon, quartz and glass are also taken into consideration. The influence of ITO microstructure on the PS interface is correlated with the electrical and luminescent behavior of the resulting heterojunction diode structure.

Photophysical properties of nano Si/SiO x composites in Al/composite/mono Si structures for green light emitting and photodetector-Schottky diodes

The concept of the self-formation of a nanocrystallite (nc-) Si/SiO x : Si z O y Al nanocomposite at the Al/oxidized porous silicon interface in the result of solid-phase processes between Al and oxidized porous Si (PS) and the influence of its composition on photophysical properties were developed and experimentally confirmed for the Si chip with optical intra-chip interconnect consisting of light emitting and photodetector diodes and alumina waveguide on oxidized PS surface with aluminum electrodes. The peculiarities of nanocomposite photophysical properties (the refractive index, photoluminiscence (PL) peak situation, PL spectrum shape in the green range) have been shown to be due to the quantum confinement effects (revealed by XPS, Raman spectroscopy) and depend on the Al presence in the nanocomposite (obtained by XPS, IR spectroscopy). The experimental confirmation of this concept is (i) the shift of the nc-Si valence band relatively to that of monocrystalline Si (c-Si) on 0.2–0.7 eV for nc-Si size in 2.5–6.5 nm range; (ii) the decrease of Si nanocrystallite size in the Al presence; (iii) the approach of the value of the refractive index of nc-Si : SiO : Si 2O 3 : Si z O y Al nanocomposite at λ=236 nm to that of porous Si with 45% porosity and (iv) the stable green PL spectra in the Si z O y Al presence in the nanocomposite.