Diamond Coating of Porous Silicon

Diamond coatings on porous silicon (PS) samples have been obtained by the hot-filament chemical vapor deposition(CVD) technique. We focused our attention on the coating morphology, showing experimentally that high quality diamond coatings may be produced with the PS sample kept at 710°C. The deposited patterns consist of polycrystalline grains with a plane interface with the PS layer.At 790°C, the quality of the coating is improved but the PS layer becomes damaged, and at 650°C the coating consists of diamond-like carbon particles. Besides the temperature, other factors such as the porosity, roughness and chemical activity of the PS layer deserve attention. We observed that one of the limiting factors of the deposition process was the high nucleation time. Two nucleation mechanisms are involved in the growth process. The first nucleation mechanism occurs on the top of the sharp PS features, subsequently to the nucleation a superficial film, and then a second nucleation mechanism occurs over this surface, which allows the growth process to continue. We also observed the presence of ablue-shift in the luminescence spectra following the coating.     

Investigation of Morphology of Porous Silicon Formed on N+ Type Silicon

The properties of porous silicon (PS) are closely connected to its morphology, much investigation has been done in order to correlate the morphological characteristics of PS with the anodisation parameters. In this paper the results of morphological analysis of PS formed on N⁺type substrates of 1 0 0 and 1 1 1 orientation are presented. The dependences of the porosity, thickness of PS, density of pores and of the effective surface on the current density are obtained. Interpretation of these results in terms of diffusion layer and energy levels is given, with special attention given to the low current density case.     

Observation of Structural Depth Profiles in Porous Silicon by Atomic Force Microscopy

The morphology of porous silicon (PS) formed by wet etching of Si crystals in fluoride solutions was investigated by atomic force microscopy (AFM). The experiments were made in a liquid cell to allow the measurements to be made before the drying process caused restructuring of the surface porosity. We studied the surface roughness, showing experimentally that PS samples produced in high HF concentrations are smoother than PS samples produced in low HF concentrations. We also demonstrated that using the capillary forces produced by the AFM probe tip itself, it is possible to etch layers of the PS material, opening windows to observe the interior PS layers. We identified through Fourier transform analysis the most frequent dimensions of the pores, concluding that these pores in general do not suffer appreciable vertical narrowing and that high HF concentrations are favorable for the formation of more pores of smaller size.     

Orientation Effect of the Wafer on the Structural Properties of P+ Type Porous Silicon Layers

The structural properties of (111) oriented p⁺ type Porous Silicon (PS) samples are investigated using various X-ray diffraction techniques and compared to (001) p⁺ type PS layer structure. High resolution X-ray diffractometry was used to record rocking curves and reciprocal space maps, giving indications about the crystalline quality of the PS samples as well as about the pore orientation. X-ray diffraction and reflectivity performed on thin PS layers allow to estimate the layer thickness, porosity and roughness of the PS/substrate interface.     

影响多孔玻璃孔结构的因素

研究表明,利用填充法制备的多孔玻璃的孔参数(气孔率、孔径分布)可进行设计与控制,多孔玻璃的气孔率和孔径分布主要取决于成孔剂的体积比及其颗粒分布,前者与后者之间的偏差取决于生坯制备及烧结过程.     

Optical Absorption in Porous Silicon

The optical properties of porous silicon (p-Si) are calculated from the electronic band structure obtained by means of an sp₃s^* tight-binding Hamiltonian and a supercell model, in which the pores are columns detched in crystalline silicon (c-Si). The disorder in the pore sizes and the undulation of the silicon wires are considered by the existence of arandom perturbative potential, which produces non-vertical interband transitions, otherwise forbidden. A typical interval around each k-vector (optical window), where non-vertical transitions make an important contribution, depends on the value of the disorder and its order of magnitude is given by l^–1, where l is the localization length. The calculated absorption spectra are compared with experiments, showing good agreement.     

Studies on Gold/Porous Silicon/Crystalline Silicon Junctions

Electrical transport in Gold/porous silicon/crystalline silicon junctions has been studied. The junctions are found to improve when the porous silicon is exposed to a hydrogen plasma before depositing the top metal. The hydrogen passivated junctions exhibited higher current levels and emitted light at lower voltages as compared to the unhydrogenated ones. Internal photoemission measurements were carried out to investigate the gold/porous silicon barrier. The barrier height determined from the Fowler plot is independent of the top material. The temperature dependence of the barrier height is similar to that of the crystalline silicon energy gap.     

Porous Silicon Studied with Time-Resolved Photoluminescence

Porous silicon has been studied with time-resolved photoluminescence, and growth as well as decay curves have been measured at several detection energies, with sample temperatures between 10 and 300 K. In the decay curves, three components are mainly observed, a small one which is very fast, with time scales of the order of nanoseconds or faster, the main component having time scales of the order of milliseconds, and a very small, very slow component, with time scales of the order of seconds. The main components can in most—but not all—cases be fitted well with stretched exponentials containing two fitting parameters. Of these, it comes out that the parameter accounting for disorder or the like depends only little upon detection energy and temperature, whereas the parameter accouting for the development in time decreases substantially for increasing temperature. The results are discussed.     

Pulsed Anodic Anodisation of Porous Silicon Films

Porous Silicon is conventionally made by dc anodisation of silicon. In this paper we have studied the luminescence of porous silicon made by pulsed anodisation as a function of duty cycle and HF concentration. Specifically we show for the first time that the luminescence can be tuned over a wide range in energy.     

An Investigation of Porous Silicon by Means of Positron Annihilation

Positron lifetime spectroscopy has been used to investigate a porous silicon film subjected to heat treatments up to 1170°C. Annealings between 300 and 500°C resulted in a 17% mass increase of the film due to oxygen uptake following the effusion of hydrogen. The positron data also indicate that vacancy clusters are formed in the silicon oxide layer or the silicon oxide—silicon interface surrounding the nanocrystallites as oxygen replaces the effusing hydrogen. The vacancy cluster concentration, which may have a bearing on the photoluminescent properties, increased by a factor of three with heating to 500°C and then decreased to one-third the original value at higher temperatures. Above 900°C vacancy migration and clustering occurred, accompanied by visible deterioration of the film.     

Light Emission from Highly Reflective Porous Silicon Multilayer Structures

Porous silicon photoluminescence and electroluminescence can be controlled by periodically modulating the material porosity to form high quality multilayer stacks and microcavities. Important issues not yet fully addressed are (a) the precise role played by this microstructuring, given that the luminescence is distributed throughout the entire structure and that the low porosity layers are highly absorbing at short wavelengths, and (b) whether the quality of such microcavities could be sufficient to support lasing. Using both experimental and theoretical techniques, the emission and reflection properties of different porous silicon single and multilayer structures have been investigated in order to understand further and exploit the nature of light propagation within them.     

Laser Induced Degradation of Photoluminescence Intensity of Porous Silicon

The evolution, under vacuum, of the photoluminescence (PL) intensity of porous silicon (PS) has been studied as function of anodisation conditions, laser line and post-anodisation treatments. It was shown that the degradation of the PL intensity depends on the internal structure of PS. In particular, the degradation is important for PS layers formed essentially by crystallites having small size or where amorphous phase exists. The experimental results have been interpreted using a theoretical model, which takes into account the variation with time of the local concentration of the luminescent centers.     

AC Conductivity of Porous Silicon from Monte Carlo Simulations

The AC conductivity of a percolation model with local energetical disorder for porous Silicon in three dimensions, (), is studied by Monte Carlo simulations. The model includes both diffusion and recombination processes and () is obtained by a Fourier transform of the mean-square displacement of the carriers, where hopping diffusion of a single type of carrier (either an electron or an exciton) and two types of carriers (an electron and a hole) are considered. It is found that at low temperatures, the behavior of () depends sensitively on the type of carrier considered.     

Self-consistent Recombination Scheme in Porous Silicon Under Intense Laser Excitation

An experimental investigation of the general characteristics of nonradiative and radiative recombination of charge carriers in strongly excited porous silicon is presented. It is shown that photoconductivity, photomagnetoelectric effect, quantum yield, and intensity of visible radiation of porous silicon demonstrates strong nonlinearities against laser excitation intensity. It is suggested that the band-to-band Auger recombination is dominant similar to that in crystalline silicon, whereas the visible luminescence is determined by the bimolecular process. The nonequilibrium density of charge carriers n 10¹⁹ cm^–3, and the bimolecular radiative recombination coefficient B_rad 9 × 10^–14 cm₃/s have been found.     

多孔陶瓷孔径及其分布测定方法研究进展

孔径及其分布决定了多孔陶瓷的性能及应用,因此对其测定和定量表征非常重要。本文综述了多孔陶瓷孔径及孔径分布的常见测定方法,包括气泡法、压汞法、气体透过法、气体吸附法、气体渗透法、液-液法、悬浮液过滤法、X射线小角度散射法、核磁共振成像法、X射线断层扫描法以及电子显微镜图像分析法。比较了各种测试方法的优缺点,认为电子显微镜图像分析法是最直接有效的测定方法,并对多孔陶瓷的测试表征方法提出了展望。     

Electrochemical Aspects of Porous Silicon Formation

The electrochemistry of porous silicon formation has been investigated by different electrochemical as well as surface analytical methods. The kinetics of pore nucleation was observed as small steps in fast current and potential pulse transients. Oxidic intermediates were identified by ex-situ XPS. Cyclic voltammetry in solutions of different HF concentration was correlated with the etching rate of silicon dioxide. On the basis of these experimental data, an electrochemical model for the porous silicon formation is presented.     

Probing Carrier Transport in Porous Silicon with Synchrotron Radiation

In this paper we demonstrate that photo-electron emission excited by X-UV synchrotron radiation can be used as a contactless probe of the gross conduction processes in porous silicon. Moreover we demonstrate that this approach reveals the underlying conduction geometry. We show that conduction in porous silicon is to some degree controlled by percolation phenomena and finally present data which support the notion that the fundamental blocking process may be Coulomb Blockade [P.A. Lee, Physica B 189, 1–5 (1993); D. Ali and H. Ahmed, Appl. Phys. Lett. 64, 2119–2120 (1994)].     

Raman Scattering from Metal-Deposited Porous Silicon

Raman scattering from porous silicon layer into which silver is immersion-plated was studied. Ag-deposited samples show extra Raman bands. Heat treatment of the Ag-deposited samples results in a great decrease in such Raman bands. Also dipping in hydrofluoric acid solution causes a spectral change. Some comments on the assignment of the Raman peaks of the Ag-deposited porous silicon are given, and the structure of porous silicon on which metal is immersion-plated is discussed.     

Porous Silicon Micromachining to Position Optical Fibres in Silicon Integrated Optical Circuits

Low-loss optical fibre connections require deep grooves etched in silicon substrate for accurate fibre positioning. As shown in this paper these grooves can be obtained by using localised formation of porous silicon on patterned substrates. Cr-Au masking layer with a duration in HF solution longer than 30 min is used to fabricate grooves with a depth higher than 75 m. N⁺-type silicon provides grooves with a pseudo-V shape which is compatible with accurate fibre alignment. By using this technology, arrays of optical fibres are positioned with an accuracy higher than 1 m.     

Raman Light Scattering in System of Oriented Wires in Porous Silicon

Raman-light scattering in porous silicon samples with oriented quantum wires was studied. It was shown, that the experimental data depends on the type of organization of wire system. The explanation of observed effect is discussed.