Development of porous silicon matrix and characteristics of porous silicon/tin oxide structures

Porous silicon (PSi) was formed at different current densities in the range of 5-60 mA/cm² by electrochemical anodized etching in HF for different durations in the range of 10-30 min. Above this PSi structure, SnO₂ films were deposited by the spin coating technique. The PSi has been characterized by X-ray diffraction studies. Peaks pertaining to PSi along with those corresponding to SnO₂ are observed. Atomic force microscopic studies indicate that very fine needle like silicon nanostructures are observed which is the result of the best PSi structure formed at 30 mA/cm². For the SnO₂ covered PSi structures, larger grains are observed with uniform coverage. The PSi samples prepared at current densities above and below 30 mA/cm² show PL spectra with asymmetric and overlapped peaks. The PL profile of thin SnO₂ film coated on PSi shows a peak at 633 nm and a small hump at about 660 nm.     

Photosensitive organic-inorganic hybrid structures based on porous silicon

Abstract

In this study, the photovoltaic organic-inorganic structures were created by deposition of poly(3,4-ethylenedioxythiophene) film doped by poly(styrenesulfonate) and reduced graphene oxide on the porous silicon/silicon substrate. Formation of the hybrid structure was confirmed by means of atomic-force microscopy and Fourier transform infrared spectroscopy. The current-voltage characteristics of the obtained structures were studied. It was found the increase of electrical conductivity and photo-induced signal in organic-inorganic structures. Temporal parameters and spectral characteristics of photoresponse in the 400–1100?nm wavelength range were investigated. The widening of spectral photosensitivity in a short-wavelength range due to light absorption in various layers of the multijunction structure in comparison with single crystal silicon was revealed.     

Study of surface and interface roughnesses in porous silicon by high-resolution X-ray methods

The methods of triple-crystal X-ray diffractometry and the total external X-ray reflection are used to study porous silicon films on a p-type single crystal Si(111) substrate. For the first time, an increase of the pseudopeak intensity was experimentally observed for thick porous films. The following film characteristics are determined: thickness (1.8 μm), strain (3.8 × 10^?3), porosity (70%), pore size (～5 nm), roughness height of the surface (～3 nm) and the film—substrate interface (～7 nm), and correlation length (～7–10 nm). It is shown that the main contribution to the pseudopeak intensity for thin films on single crystal substrates comes from angular broadening of the incident beam formed by the exit slit of a monochromator of a finite width. It is shown that the method is very sensitive to density inhomogeneity in subsurface crystal layers.     

An X-ray scattering study of hydrogenated nanocrystalline silicon with varying crystalline fraction

Using X-ray diffraction (XRD) and small angle X-ray scattering (SAXS), we probed the nanostructural features of several PECVD grown nc-Si:H thin films with varying crystalline volume fraction. XRD results of a mixed phase film, 70% a-Si:H and 30% c-Si:H, show these crystallites have a preferred [220] orientation in the growth direction. Another film with approximately 90% c-Si also shows elongated grains, but with a preferred [111] orientation. The SAXS results also show an increase in scattering intensity when compared to the mixed phase material. In the mixed phase material, models show that the electron density fluctuations between the amorphous and crystalline phases are not enough to explain the measured SAXS scattering. Hydrogen clustered at the crystallite boundaries and in void regions of the a-Si phase must be included as well.     

X-ray reflectivity studies of very thin films of silicon oxide and silicon oxide–silicon nitride stacked structures

Oxide/nitride/oxide films were deposited onto bare silicon substrates by low-pressure chemical vapor deposition (LPCVD) of silicon nitride and thermal oxidation. The X-ray reflectivity technique has been used to study the influence of the growth conditions and of different cleaning procedures of the silicon substrate on the structure and morphology of the deposited multilayers. The results revealed how, from an analysis of the X-ray reflectivity data performed by using Fresnel equations for multilayers modified to account for the interface imperfections, we determine, in a non-destructive manner, structural parameters such as density, thickness, roughness, and interface structure of the whole dielectric layers, giving us more information and greater sensitivity respect to cross-section transmission electron microscopy (TEM) and ellipsometric measurements.     

Strong and stable red photoluminescence from porous silicon prepared by Fe-contaminated silicon

Strong red photoluminescence (PL) spectra appeared at porous silicon (PS) samples prepared by a chemical anodization of Fe-contaminated Si substrates. The Fe1000 sample with Fe contamination of 1000 ppb showed a ten times stronger red PL than that of the reference PS sample without any Fe contamination, and this sample also showed the higher thermal stability for PL spectra as compared with the reference PS sample. Furthermore, the PL intensity from the PS with Fe contamination is linearly proportional to the Fe-related trap concentrations of Si substrates obtained from DLTS. Especially, all the PS samples exhibit the same PL peak position regardless of Fe contamination concentrations, as compared with that of the reference PS. This means that there is no significant effect such as the variation of size distribution of nanocrystalline Si in PS layer formed on Fe-contaminated Si substrate. Based on the results of PL and DLTS, we found that the PL efficiency depends strongly on the Fe-related trap concentration in Si substrates.     

Formation and oxidation of porous silicon by anodic reaction

Porous silicon film (PSF) was formed by anodic reaction in aqueous hydrofluoric acid (HF) in the range 2 ? n < 4, n being the average number of electrons flowing through the external circuit per atom of silicon dissolved. Electron diffraction pattern of as-grown PSF changes to a very broad Kikuchi pattern from a sharp Kikuchi pattern for smaller values of n. The disproportionation reactions occur at a local point of the silicon surface for n between 2 and 4. When n is nearly equal to 2, the PSF structure changes to be amorphous and the reactions occur over the entire area of the silicon surface. The oxidation of PSF proceeds with the increase of the absorption intensities of three Si-O bands at 1070, 455, and 800 cm^-1 below 600°C, and two Si-O bands which are 455 and 800 cm^-1 above 600°C. Activation energies of Si-O bands at 1070, 455, and 800 cm^-1 at oxidation below 600°C are 0.1, 0.3, and 0.6 eV respectively. The activation energy changes remarkably below and above 600°C. The oxygen distributions in oxidized porous silicon of p-type and n-type PSF are uniform and nonuniform respectively in the thickness direction.     

Investigation of the tripoli porous structure by small-angle neutron scattering

The characteristics of the tripoli porous structure have been investigated by small-angle neutron scattering (SANS). Tripoli is a finely porous sedimentary rock formed by small spherical opal particles. Its main component is aqueous silica SiO₂ · nH₂O (80–90%). Tripoli is widely used in practice as a working medium for sorption filters and in some other commercial and construction technologies. The shape of the experimental SANS curves indicates the presence of small and large pores in tripoli. The small-pore size was estimated to be ～100 Å. The size of large pores turned out to be beyond the range of neutron wave vector transfers Q that are available for the instrument used; however, their size was indirectly estimated to be ～(2000–2500) Å. The pores of both groups behave as surfacetype fractal scatterers with the fractal dimension D ～ 2.2‐2.6. The densities of pores of these two groups differ by approximately three orders of magnitude (～10¹⁶ and ～10¹³ cm^?3 for small and large pores, respectively); the fraction of large pores amounts to 70–80% of the total pore volume. The found pore characteristics (their densities, sizes, and relative volumes) are in satisfactory agreement (when a comparison is possible) with the absorption data.     

RF plasma treatment of porous silicon

The effect of oxygen and hydrogen rf plasmas on the photoluminescence (PL) spectra of porous silicon (PS) was studied. PS samples were prepared from p-type, (100) oriented silicon wafers with a resistivity of 0.03 Ω cm by electrochemical anodization in an ethanol-containing solution. The plasma treatments were carried out in a planar reactor for 150 min, whereas the samples were placed on the ground electrode and heated to 250°C. The PL was excited by the 548 nm line of Hg lamp and measured using a monochromator and lock-in amplifier. Both hydrogen and oxygen plasma treatments led to a decrease in PL intensity. The decrease was with a factor of 5 after hydrogen and 2.5 after oxygen plasma treatment. It was established by means of Auger electron spectroscopy (AES) that both plasma treatments led to an increase in the oxygen content and to a decrease in the carbon content on the PS surface. However, no direct correlation between the AES data and the PL intensity could be established. We suggest that the appearance of plasma-induced electronic defects, rather than the change in the surface chemistry, accounts for the PL quenching.     

Structure of natural film on porous silicon surface founded by ellipsometry

Abstract

The multiple-angle-of-incidence ellipsometric studies of the natural film formed on the surface of porous silicon sample after its production and subsequent keeping in isopropyl alcohol have been performed. A new property of porous silicon with the natural film on its surface has been revealed which consists in a drastic change of the polarization characteristics of the reflected light wave after its keeping in alcohol. This property of porous silicon may be the basis for the development of liquid and gas sensors, polarization elements of optical devices etc. It was found that ellipsometric curves are described better by calculations in a two-layer model in comparison with a single-layer one of the investigated film. It was obtained that the outer layer of the film has smaller refractive index than the inner one. As a result of decoration the refractive indices of both layers decrease and further the changes of refractive indices and the redistribution of the thickness of both layers are found. It is suggested that the interaction of the alcohol molecules with silicon and its oxide environment occurs in the walls of the pores during the decoration.     

X-ray diffraction analysis of modulated crystals: Review

The state of the art of X-ray diffraction analysis of modulated crystals is reviewed. The review begins with a brief historical overview followed by the consideration of the main concepts and notations used in this field. Then, methods of structural analysis of modulated crystals are considered with emphasis on recent achievements. The most interesting objects are listed, and the directions of investigation are outlined. Examples of analysis of both individual structures and families of modulated and incommensurate composite structures are given in terms of superspace symmetry.     

Electronic structures of silicon nitride revealed by tight binding calculations

A new tight binding (TB) potential model was proposed for determining the electronic structures of ionic-covalent materials. In the TB model, the matrix elements were determined from the atomic characteristics of the crystal. The atomic parameters of the solid were determined based on the general quantum principles and no adjustable parameter was needed. Electronic structures of amorphous silicon nitride (Si₃N₄) were calculated using this method. A good agreement between the calculated and experimental values in terms of fundamental properties such as the position of the valence-band edge, the conduction-band edge, and the energy bandgap were obtained. Charge transfer between the silicon and nitrogen atoms was also precisely calculated in this work.     

Crystalline and amorphous nanostructures in porous silicon

We report ab initio molecular dynamic simulations of several supercells of crystalline porous silicon, that are first relaxed and then analyzed by their radial distribution functions (RDF). The porosities vary from 10% to 80% of the total volume of the supercell. The interatomic distance is determined by the position of the first peak of the RDF. We manipulated a maximum of 500 atoms of silicon and a minimum of 32. The interatomic distance of the model with a porosity of 10% was 2.35 Å, for those models with porosity from 11% to 50% was 2.45 Å and finally, 2.55 Å for those with a porosity greater than 50%. If the supercell backbone structure is small compared with the void of the supercell, then the interatomic distance between the silicon atoms run out of the crystalline value. Our results agree with experiment.     

Microstructure and composition analysis of group III nitrides by X-ray scattering

The importance of Group III-nitride structures for both light-emitting devices and high-power field effect transistors is well known (J.W. Orton, C.T. Foxon, Rep. Prog. Phys. 61 (1998) 1). In both cases, different alloy composition and doping levels or type are utilised and the device performance also depends critically on the interface quality and defect density. We have used high resolution X-ray scattering to measure the state of strain in the individual layers on an absolute scale to derive the alloy composition, i.e. we have avoided the conventional method of using the substrate as an internal reference since it could be strained. The composition and individual layer thickness are derived through simulation of the profile with this additional strain information and the best-fit profile is obtained with an automatic procedure. These structures are laterally inhomogeneous arising from defects breaking up the structure into narrow vertical columns of nearly perfect material and this produces significant broadening of the diffraction pattern. This broadening in the diffraction pattern has been modelled using an extended dynamical scattering model (P.F. Fewster, X-Ray Scattering from Semiconductors, Imperial College Press, World Scientific, Singapore, 2000) to yield the size distribution of perfect crystal regions. The measurement of the rotation about an axis defined by the growth direction of the GaN with respect to the sapphire is determined and is found to be small. However, a poor quality sample indicates that a large range of rotations is possible in these structures.     

The structure of phosphate glass evidenced by small angle X-ray scattering

Numerous phosphate glass systems with compositions covering the entire glass forming range have been examined by small angle X-ray scattering. The experiments were carried out in a widely extended interval of s values between 0.055 and 31 nm⁻¹. A small angle scattering effect was detected for all samples investigated with the exception of the aluminium metaphosphate glass. The small angle scattering recorded indicates the presence of two differently-sized heterogeneity regions of electron density. The small-scale heterogeneities are present in magnesium and zinc phosphate glasses only. Their size is about 1 nm diameter in the magnesium and about 2 nm in the zinc phosphate glasses. The species do not result from liquid–liquid phase separation. Examinations to extract information on the nature of these heterogeneity regions are described in detail. There is no fundamental knowledge on the large-scale heterogeneities established. The small angle scattering suggests that their occurrence is related to water contamination. Anomalous small angle X-ray scattering experiments of a series of zinc polyphosphate glasses and a strontium metaphosphate glass sample are performed to examine the distribution of the Zn or Sr atoms, respectively.     

Characterization of the structure of porous germanium layers by high-resolution X-ray diffractometry

The surface morphology and the structure of porous germanium layers obtained by chemical etching of n-type single-crystal Ge(111) substrates with their subsequent annealing in hydrogen atmosphere are studied by high-resolution X-ray diffractometry. It is established that upon etching a 1.5 to 2.0-μm-thick porous germanium layer is formed, which contains quasi-ordered microinhomogeneities in the form of elongated pits with characteristic dimensions of 1 μm and an average distance between them of 3–4 μm. The layer bulk has pores with radii ranging within 25–30 nm and nanocrystallites with an average size of 10 nm, with the average porosity being 56%.     

Characterization of porous InP(001) layers by triple-crystal X-ray diffractometry

The structure of porous layers fabricated on the surface of an InP(001) semiconductor wafer by electrochemical oxidation has been studied by triple-crystal X-ray diffractometry. It is shown that, depending on the probe potential (current density), column-shaped pores of two types with different orientations with respect to the substrate are formed. It is established that, at a probe potential of ～6 V, pores are extended along the current lines (CLO pores) and directed normally to the sample surface. With a decrease in the potential to ～3 V, the longitudinal axes of pores are oriented at an angle of ζ = 52° ± 2° to the sample surface (CO pores). The average diameter and length of pores are determined (～80 and 300–900 nm, respectively) and the porosity of the samples is evaluated (P ～ 40–60%). The obtained data on the pore parameters are in satisfactory agreement with the scanning electron microscopy data.     

Hydrogen and oxygen concentration analysis of porous silicon

Elastic recoil detection and Rutherford backscattering spectrometry combined with the nuclear reaction analysis method have been applied for the determination of oxygen, hydrogen and carbon concentration depth profiles of aged p⁺-type porous Si layers of different low and medium porosities. The plan view and cross-section scanning electron microscopy images have provided with information about both the diameter and silicon skeleton structure of the pores. The concentration depth profiles reveal the existence of a non-homogeneous subsurface porous film several hundred nanometers thick for all the studied samples. Differences in the atomic composition among low and medium porosity layers and the possible origin of various impurities are discussed. The maximum H content in PSi has been observed at the depth of 200–600 nm, while the highest oxygen concentration is typical of 200 nm thick subsurface layers. The highest obtained ratio of H/Si atomic concentrations reaches the value of 2 for the PSi samples with porosity P of 66%, comparing to N_H/N_Si = 0.27 in the case of P = 25% PSi.     

Controlled light emission from dye-impregnated porous silicon microcavities

The properties of porous silicon Fabry–Perot microcavities impregnated with oxazine, a fluorescent dye, are investigated by photoluminescence and reflectance spectroscopy. The emission spectrum of the dye is noticeably modified by the cavity structures, being both the peak intensity increased and the linewidth narrowed, while the peak position coincides with that measured by UV–Vis reflectance. The results suggest that is possible, by simple methods, to prepare optically active hybrid materials integrating the properties of the inorganic host structure and of the organic guest.