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In this article, cadmium oxide (CdO) was deposited using pulsed laser deposition approach on porous silicon (Si) wafer for visible light photodetector application, through which a series of devices were proposed as a function of the deposition energy. The microstructural as well as optical characteristics of the prepared film/s were demonstrated, respectively, using x-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and ultraviolet visible light spectroscopy (UVâ€"Vis) analysis. In details, the UVâ€"Vis analysis revealed the occurrence of optical band gaps within the range of 2.38â€"2.42 eV , while an average nanoparticle diameter was found to be 45 nm using FE-SEM technique. This in turn demonstrated a sound relation with the photoresponsive behavior of the attained photodetectors. A photoresponsivity and specific detectivity of 1.9 ÎĽ A m W âˆ' 1 and 1.21 Ă— 10 9 Jones were attained using 700 mJ laser energy. In the meanwhile, the estimated response/recover time of the addressed laser energy was found to be 300 s and 340 s , respectively. The photo-responsive characteristics of the fabricated devices were found to be in positive linear correlation with the applied laser energy. [ABSTRACT FROM AUTHOR]

In this paper, we are reporting on the fabrication of a porous silicon/Au and silicon filament/Au using the two-step Au-assisted chemical etching of p-type Si with a specific resistivity of 0.01, 1, and 12 Ω·cm when varying the Au deposition times. The structure analysis results show that with an increasing Au deposition time of up to 7 min, the thickness of the porous Si layer increases for the same etching duration (60 min), and the morphology of the layer changes from porous to filamentary. This paper shows that the uniform macro-porous layers with a thickness of 125.5â€"171.2 ÎĽ m and a specific surface area of the mesopore sidewalls of 142.5â€"182 m2·gâˆ'1 are formed on the Si with a specific resistivity of 0.01 Ω·cm. The gradient macro-porous layers with a thickness of 220â€"260 ÎĽ m and 210â€"290 ÎĽ m, the specific surface area of the mesopore sidewalls of 3.7â€"21.7 m2·gâˆ'1 and 17â€"29 m2·gâˆ'1 are formed on the silicon with a specific resistivity of 1 and 12 Ω·cm, respectively. The por-Si/Au has excellent low-temperature electro oxidation performance with ethanol, the activity of ethanol oxidation is mainly due to the synergistic effect of the Au nanoparticles and porous Si. The formation mechanism of the uniform and gradient macro-porous layers and ethanol electro-oxidation on the porous/filament silicon, decorated with Au nanoparticles, was established. The por-Si/Au structures with perpendicularly oriented pores, a high por-Si layer thickness, and a low mono-Si layer thickness (with a specific resistivity of 1 Ω·cm) are optimal for an effective ethanol electro-oxidation, which has been confirmed with chronoamperometry measurements. [ABSTRACT FROM AUTHOR]

The present study demonstrates the tuning of optical characteristics of porous silicon (PSi)-based microcavities by N ion beam interactions. These optical microcavities are prepared by using electrochemical etching of heavily doped p+-type Si. The PSi microcavities were exposed to N ions of 200 keV and 1 MeV at an optimized ion fluence of 1 Ă— 1015 ions cmâˆ'2. A significant red-shifting of 32 ∼ 60 nm in the resonance cavity mode was observed due to ion interaction. The experimental results are in good agreement with the transfer matrix simulations. A substantial modification of the PSi microcavity surface states is visualized through Raman and x-ray photoelectron spectroscopy (XPS) techniques. The Raman spectral results show modifications from crystalline Si to nanostructured Si and subsequently to amorphous Si. The XPS indicates the modification of Siâ€"Si and Siâ€"O bonds and the formation of new Siâ€"N bonds, implying the presence of Si3N4. These experimental observations, along with analytical simulations and transfer-matrix method microcavity modeling, conclusively support the realization of cavity tunability and substantial modification in the optical field intensity and photon confinement within the spacer layer of the microcavity. These results suggest that ion beams are the effective tool to produce wider tunable optical properties in microcavities with highly stable designer optical structures suitable for photonic applications. [ABSTRACT FROM AUTHOR]

Due to its high specific capacity, silicon is a promising candidate to substitute conventional graphite as anode material in lithiumion batteries. However, pure silicon-based anodes suffer from poor capacity retention, mainly due to a large volume change during cycling, which results in material pulverization and other side reactions. Therefore, alternative compositions with lowered silicon content and a similar working voltage as graphite are favored, e.g. silicon-graphite (SiG), as they can reduce these volume change and side reactions while maintaining a high capacity. Here, neutron depth profiling (NDP) offers the unique possibility to quantify non-destructively the lithium concentration profile over the depth of these electrodes. In this study, the (de-)intercalation phenomena during (de-)lithiation in SiG porous anodes with silicon contents ranging from 0 wt% to 20 wt% is investigated for the first time using ex situ NDP during the initial discharge at defined depths of discharge (DODs) states. These findings are complemented by a conventional electrochemical analysis of the first full cycle with a charge/discharge rate of C/20. While the specific capacity is observed to increase with higher silicon content, NDP directly reveals a homogeneous irreversible lithium accumulation within the entire electrode depth. [ABSTRACT FROM AUTHOR]

We investigate a graphene-coated nanowire waveguide (GCNW) composed of two suspended wedge porous silicon nanowires and a thin Ag partition. The plasmonic characteristics of the proposed structure in terahertz (THz) frequency band are simulated by the finite element method (FEM). The parameters including the gap between the nanowires and Ag partition, the height of the nanowire, the thickness of the Ag partition, and the Fermi level of graphene, are optimized. The simulation results show that a normalized mode field area of ∼ 10−4 and a figure of merit of ∼ 100 can be achieved. Compared with the cylindrical GCNW and isolated GCNW, the proposed wedge GCNW has good electric field enhancement. A waveguide sensitivity of 32.28 is obtained, which indicates the prospects of application in refractive index (RI) sensing in THz frequency band. Due to the adjustable plasmonic characteristics by changing the Fermi level (EF), the proposed structure has promising applications in the electro-optic modulations, optical interconnects, and optical switches. [ABSTRACT FROM AUTHOR]

One-pot Ag-assisted chemical etching (SACE) of silicon provides an effective, simple way to obtain Si nanowires (NWs) of potential interest for technological applications ranging from photovoltaics to thermoelectricity. The detailed mechanism ruling the process has not been yet fully elucidated, however. In this paper we report the results of an extended analysis of the interplay among doping level and type of silicon, nanowire nanomorphology and the parameters controlling the chemistry of the etching process. We provide evidence that the SACE mechanism entirely occurs at the interface between the etching solution and the Si substrate as a result of Si extrusion by sinking self-propelled Ag particles. Also, a rationale is advanced to explain the reported formation of (partially) porous NWs at high doping levels in both p- and n-type Si. A model not relying on the asserted formation of potential barriers enables to recover full consistency between SACE electrochemistry and the mechanism of formation of porous silicon in electrochemical cells. [ABSTRACT FROM AUTHOR]

Composites of nanostructured porous silicon and silver (nPSi-Ag) have attracted great attention due to the wide spectrum of applications in fields such as microelectronics, photonics, photocatalysis and bioengineering, Among the different methods for the fabrication of nanostructured composite materials, dip and spin-coating are simple, versatile, and cost-effective bottom-up technologies to provide functional coatings. In that sense, we aimed at fabricating nPSi-Ag composite layers. Using nPSi layers with pore diameter of 30 nm, two types of thin-film techniques were systematically compared: cyclic dip-coating (CDC) and cyclic spin-coating (CSC). CDC technique formed a mix of granular and flake-like structures of metallic Ag, and CSC method favored the synthesis of flake-like structures with Ag and Ag2O phases. Flakes obtained by CDC and CSC presented a width of 110 nm and 70 nm, respectively. Particles also showed a nanostructure surface with features around 25 nm. According to the results of EDX and RBS, integration of Ag into nPSi was better achieved using the CDC technique. SERS peaks related to chitosan adsorbed on Ag nanostructures were enhanced, especially in the nPSi-Ag composite layers fabricated by CSC compared to CDC, which was confirmed by FTDT simulations. These results show that CDC and CSC produce different nPSi-Ag composite layers for potential applications in bioengineering and photonics. [ABSTRACT FROM AUTHOR]

In this work, a porous silicon nanostructure has been fabricated by electrochemical means and used as a thermal sensor. The thermo-optic effect in the near infrared region has been experimentally studied based on spectroscopy measurements. Values of the thermo-optic coefficient between 3.2 and 7.9·10-5 K-1 have been obtained, depending on the porosity, reaching a maximum thermal sensitivity of 91 ± 3 pm/°C during the experiments carried out with the fabricated samples. Additionally, the oxidation process of the sensor at temperatures below 500 K has been studied, showing that the growth of the silicon oxide was dependent on the characteristics of the porous layers. Based on the experimental results, a mathematical model was developed to estimate the evolution of the oxidation process as a function of porosity and thickness. [ABSTRACT FROM AUTHOR]

Silicon-graphite (Si/C) composite anodes are used to increase total anode capacity while maintaining a tolerable degree of active material volume expansion. However, increasing the Si/C ratio does not directly lead to an increase in the accessible capacity because excessive volume expansion can lead to unacceptable cell pressure or electrode porosity. To predict the accessible capacity as a function of Si/C ratio, we integrated mechanical behavior for individual cell components into our previous battery model that couples mechanical and electrochemical phenomena and then simulated a full charge of a pouch cell with foam packing. The simulations were used to determine the anode accessible capacity as a function of Si/C ratio, based on practical pressure and porosity design limitations. The resulting predictions illustrate the tradeoff between the capacity gained by increasing the Si/C ratio and the accessible capacity lost based on the pressures that build up in the cell due to the interconnected mechanical and electrochemical phenomena. For a given set of cell and pack design requirements, battery designers can use these types of simulations as a tool to maximize accessible capacity of an operating cell by selecting appropriate cell/pack materials and identifying optimal Si/C ratios. [ABSTRACT FROM AUTHOR]

Tuning the pore diameter of porous silicon films is essential for some applications such as biosensing, where the pore size can be used for filtering analytes or to control the biofunctionalization of its walls. However, macropore (>50nm) formation on p-type silicon is not yet fully controlled due to its strong dependence on resistivity. Electrochemical etching of heavily doped p-type silicon usually forms micropores (<5nm), but it has been found that bigger sizes can be achieved by adding an organic solvent to the electrolyte. In this work, we compare the results obtained when adding dimethylformamide (DMF) and dimethylsulfoxide (DMSO) to the electrolyte as well as the effect of a post-treatment of the sample with potasium hydroxide (KOH) and sodium hydroxide (NaOH) for macropore formation in p-type silicon with resistivities between 0.001 and 10Ω· cm, achieving pore sizes from 5 to 100nm. [ABSTRACT FROM AUTHOR]

This paper presents experimental results on the creation of porous silicon structures with variable pore morphology (GPS-var). At a pore depth of up to several hundred microns, the structures are characterized by the presence of a layer with nanoscale porous structures (hereinafter, nanoporous) on the surface. As the depth of etching increases, the pores smoothly transform into spongy, and then into a columnar structures with pore sizes of about a micron. [ABSTRACT FROM AUTHOR]

Combination of conventional electrochemical etching and subsequent metal-assisted chemical etching (MACE) was used to obtain double etched porous silicon (DEPSi) structures with good optical properties and high affinity to bacteria. Second etching step increased the roughness of the surface and created more cavities for entrapping of bacteria, while optical scattering on the surface was still insufficient and quality of infrared Fabry-Perot interferograms remained high. Obtained DEPSi structures were used for detection of bacteria (E. Coli). Fast Fourier transform of infrared spectra showed reversible broadening of the main band, what allows to detect bacteria in concentrations down to 104 CFU per mL. [ABSTRACT FROM AUTHOR]

Electrochemical deposition of Cu on structured silicon including groves and pores is critical for several applications such as microelectronics, sensors, and energy applications. In the case of Cu filling of porous Si, the silicon skeleton can be either etched to produce free nano- or micro-structured Cu or can be left to form a nano- or micro-composite of Si/Cu. Deposition of metals into n-type porous Si is a relatively straight-forward process, due to the availability of electrons at the conduction band to reduce Cu cations. Cu deposition into p-type porous Si requires illumination, unfortunately, it enhances the growth on pore walls and results in either non-conformal growth or plug-in which lead to voids. In this work, it is shown that illuminating the back side of the wafer combined with insulating the pore walls and pore top leads to conformal growth of Cu from the pore-tip to the pore-top. Two approaches have been tested: wall thinning by chemical etching and SiO2 coating on the pore top. Massive wires have been obtained by completely and conformally filling porous Si without leaving gaps in the case of SiO2-coated samples. The conformal filling of p-type Si was obtained without using additives and without seed layer. [ABSTRACT FROM AUTHOR]

Electrochemical impedance spectroscopy is used to study the etching of p-type silicon in ethanolic HF over the potential region where porous silicon forms. The impedance data fits a single equivalent circuit for all HF concentrations. The appearance of a finite length Warburg element at potentials negative of the flatband potential is interpreted as evidence that a submonolayer of oxide/hydroxide islands form in the Tafel region. Although this is contrary to previous studies it is consistent with published spectroscopy data. Low values for the double layer capacitance are attributed to the hydrophobic nature of the Si-H terminated surface, whereas an inductance loop or negative differential capacitance behavior seen at low frequencies is interpreted in terms of a potential-dependent surface roughness through the formation/dissolution of a submonolayer oxide. Interpretation of the resistive components of the circuit are in line with the known mechanism of etching, that is charge transfer changing to mixed control as the potential is made more positive and dissolution becomes limited by mass transport processes. [ABSTRACT FROM AUTHOR]

Vertically high-density ZnO nanorods were successfully synthesized on a porous silicon (PS) substrate by chemical bath deposition method. The structural and optical investigations revealed that the ZnO nanorods grown on the PS substrate had high structural and optical quality. The photoelectric properties of the fabricated photodetector were investigated with 325 nm UV light illumination under 1 V bias voltage. Based on the current–voltage curve, the responsivity of the ZnO nanorod photodetector was 1.738 A W−1 at 1 V bias voltage. Under a bias voltage of 1 V, the sensitivity of the ZnO nanorod device was 20. The response and recovery time of the ZnO nanorod photodetector under these conditions were 0.032 and 0.041 s, respectively. [ABSTRACT FROM AUTHOR]

Silicon is of great interest for use as the anode material in lithium-ion batteries due to its high capacity. However, certain properties of silicon, such as a large volume expansion during the lithiation process and the low diffusion rate of lithium in silicon, result in fast capacity degradation in limited charge/discharge cycles, especially at high current rate. Therefore, the use of silicon in real battery applications is limited. The idea of using porous silicon, to a large extent, addresses the above-mentioned issues simultaneously. In this review, we discuss the merits of using porous silicon for anodes through both theoretical and experimental study. Recent progress in the preparation of porous silicon through the template-assisted approach and the non-template approach have been highlighted. The battery performance in terms of capacity and cyclability of each structure is evaluated. [ABSTRACT FROM AUTHOR]

Porous silicon nanowire is fabricated by a simple electrospinning process combined with a magnesium reduction; this material is investigated for use as an anode material for lithium rechargeable batteries. We find that the porous silicon nanowire electrode from the simple and scalable method can deliver a high reversible capacity with an excellent cycle stability. The enhanced performance in terms of cycling stability is attributed to the facile accommodation of the volume change by the pores in the interconnect and the increased electronic conductivity due to a multi-level carbon coating during the fabrication process. [ABSTRACT FROM AUTHOR]

Silicon metal-assisted chemical etching (MACE) is a nanostructuring technique exploiting the enhancement of the silicon etch rate at some metal-silicon interfaces. Compared to more traditional approaches, MACE is a high-throughput technique, and it is one of the few that enables the growth of vertical 1D structures of virtually unlimited length. As such, it has already found relevant technological applications in fields ranging from energy conversion to biosensing. Yet, its implementation has always required metal patterning to obtain nanopillars. Here, we report how MACE may lead to the formation of porous silicon nanopillars even in the absence of gold patterning. We show how the use of inhomogeneous yet continuous gold layers leads to the generation of a stress field causing spontaneous local delamination of the metal-and to the formation of silicon nanopillars where the metal disruption occurs. We observed the spontaneous formation of nanopillars with diameters ranging from 40 to 65 nm and heights up to 1 μm. Strain-controlled generation of nanopillars is consistent with a mechanism of silicon oxidation by hole injection through the metal layer. Spontaneous nanopillar formation could enable applications of this method to contexts where ordered distributions of nanopillars are not required, while patterning by high-resolution techniques is either impractical or unaffordable. [ABSTRACT FROM AUTHOR]

Porous Si thermal conductivity was determined in a wide temperature range from 20 to 350K using the steady-state direct current method and a subsequent finite element method. The method was applied to a 40μm thick porous Si layer of ∼60% porosity, formed on p-type Si of resistivity 1–10Ωcm. It was found that in the temperature range 20–80K the thermal conductivity of the studied porous Si layer was more than four orders of magnitude lower than that of bulk crystalline Si, due to phonon confinement in the nanostructured skeleton composing porous Si. The temperature distribution as a function of distance from a heater on the porous Si layer and as a function of depth from the porous Si/metal interface was also calculated for different values of the applied electric power and for two different temperatures, 20 and 290 K, using a combination of experimental results and simulations. The effectiveness of local thermal isolation from the Si substrate by a thick highly porous Si layer in a wide temperature range was demonstrated, showing that this material is challenging for use in Si micro-cooling devices. [ABSTRACT FROM AUTHOR]

We have fabricated porous silicon nanopillar arrays over large areas with a rapid, simple, and low-cost technique. The porous silicon nanopillars show unique longitudinal features along their entire length and have porosity with dimensions on the single-nanometer scale. Both Raman spectroscopy and photoluminescence data were used to determine the nanocrystallite size to be <3 nm. The porous silicon nanopillar arrays also maintained excellent ensemble properties, reducing reflection nearly fivefold from planar silicon in the visible range without any optimization, and approaching superhydrophobic behavior with increasing aspect ratio, demonstrating contact angles up to 138°. Finally, the porous silicon nanopillar arrays were made into sensitive surface-enhanced Raman scattering (SERS) substrates by depositing metal onto the pillars. The SERS performance of the substrates was demonstrated using a chemical dye Rhodamine 6G. With their multitude of properties (i.e., antireflection, superhydrophobicity, photoluminescence, and sensitive SERS), the porous silicon nanopillar arrays described here can be valuable in applications such as solar harvesting, electrochemical cells, self-cleaning devices, and dynamic biological monitoring. [ABSTRACT FROM AUTHOR]

Measurements of contact angle of ethanol-water solutions were performed on crystalline silicon and on mesoporous silicon films with porosities up to ∼72%. Water contact angles of 44° and 76° were measured for untreated and HF-dipped crystalline silicon, respectively, consistent with previous studies. The contact angle for ethanol-water mixtures was found to decrease with increasing ethanol concentration for both untreated crystalline silicon and also for HF-dipped crystalline silicon up to an ethanol concentration of ∼80%; at higher concentrations the contact angle approached zero. Similar behaviour was observed on mesoporous silicon surfaces for ethanol concentrations ≲40%, above which the contact angle dropped abruptly to an immeasurably small value. This behaviour is attributed to a decrease in surface tension with increasing ethanol concentration. For all ethanol-water solutions, a minimum value of contact angle was observed at a porosity of ∼40%, above which it remained approximately constant. The behaviour of contact angle with porosity can be explained by a change in the Wenzel roughness parameter due to changes in the specific surface area of the film [ABSTRACT FROM AUTHOR]

We have studied oxidized porous silicon (OPS) properties using Fourier transform infraRed (FTIR) spectroscopy and capacitance-voltage C-V measurements. We report the first experimental determination of the optimum porosity allowing the elaboration of high-density OPS insulators. This is an important contribution to the research of thick integrated electrical insulators on porous silicon based on an optimized process ensuring dielectric quality (complete oxidation) and mechanical and chemical reliability (no residual pores or silicon crystallites). Through the measurement of the refractive indexes of the porous silicon (PS) layer before and after oxidation, one can determine the structural composition of the OPS material in silicon, air and silica. We have experimentally demonstrated that a porosity approaching 56% of the as-prepared PS layer is required to ensure a complete oxidation of PS without residual silicon crystallites and with minimum porosity. The effective dielectric constant values of OPS materials determined from capacitance-voltage C-V measurements are discussed and compared to FTIR results predictions. [ABSTRACT FROM AUTHOR]

The NO2 gas sensing behavior of porous silicon (PS) is studied at room temperature with and without ultraviolet (UV) light radiation. The PS layer is fabricated by electrochemical etching in an HF-based solution on a p+-type silicon substrate. Then, Pt electrodes are deposited on the surface of the PS to obtain the PS gas sensor. The NO2 sensing properties of the PS with different porosities are investigated under UV light radiation at room temperature. The measurement results show that the PS gas sensor has a much higher response sensitivity and faster response-recovery characteristics than NO2 under the illumination. The sensitivity of the PS sample with the largest porosity to 1 ppm NO2 is 9.9 with UV light radiation, while it is 2.4 without UV light radiation. We find that the ability to absorb UV light is enhanced with the increase in porosity. The PS sample with the highest porosity has a larger change than the other samples. Therefore, the effect of UV radiation on the NO2 sensing properties of PS is closely related to the porosity. [ABSTRACT FROM AUTHOR]

We report the theoretical comparison of the omnidirectional photonic band gap (OPBG) of one-dimensional dielectric photonic structures, using three different refractive index profiles: sinusoidal, Gaussian and Bragg. For different values of optical thickness, the tunability of the OPBG of each profile is shown to depend on the maximum, minimum and the difference of the refractive indices. An experimental verification was carried out with a multilayered dielectric porous silicon structure for all the three profiles. The optimal combination of refractive indices for each structure which generates the maximum OPBG has also been calculated. Such study can be useful to design the required OPBG structures for photonic applications. [ABSTRACT FROM AUTHOR]

Porous Si prepared by the magnesiothermic reaction was used as anode material for lithium ion batteries. The synthesis conditions were investigated to obtain high purity of silicon phase. The structures of porous Si were characterized by TEM, Raman and N2 adsorption-desorption. The electrochemical performance was tested by cyclic voltammetry (CV) and discharge-charge measurement. The porous Si exhibits larger storage capacity and improved cyclability compared to commercial Si powders. The enhanced capacity and cyclability are attributed to the loose mesoporous structure of porous Si, which can accommodate large volume change in the lithiation/delithiation process. [ABSTRACT FROM AUTHOR]

Porous silicon (PS) conductometric gas sensors are used to create a sensitivity matrix for the room temperature detection of NOx (NO, NO2). "P-type" nanopore coated microporous silicon is treated with tin, nickel, copper, and gold, electrolessly deposited onto the PS surface to form SnOx, NiO, CuxO, and AuxO nanostructured centers as confirmed by XPS measurements. The relative sensitivities of these modified PS gas sensor surface sites have been measured under 1-5 ppm NO exposure. An improved sensitivity of up to 10 times that of untreated PS is observed for a 1 ppm exposure. The choice of deposits is based on the hard to soft acid character of the nanostructured metal oxide islands that are fractionally deposited on the semiconductor interface and their effect on the physisorption of NO, a weak base, dictated by an inverse pattern (IHSAB) to the hard-soft acid base concept. NO, a free radical, can interact with oxygen sites on the modified PS sensor interfaces, to effect a transient NO2 signal unique to PS-based NO sensors, which is not observed as other basic analytes including NH3, PH3, H2S, SO2, and CO interact with "p-type" PS.A comparison is made between the current PS sensor systems which operate at room temperature and electrochemical and traditional metal oxide sensors. [ABSTRACT FROM AUTHOR]

Silicon On Porous Layer (SOPL) substrate was fabricated by applying the Smart Cut technology to silicon substrate having a superficial porous Si layer. It consists in a non-porous/porous single crystalline Si bilayer transferred onto a handling wafer to form Silicon-On-Insulator (SOI) like structure. Subsequent bonding to a final wafer and separation induced in the fragile embedded porous Si layer led to the double transfer of the single crystalline non porous Si layer. Changes in structural and mechanical properties of the embedded porous silicon layer as a function of the annealing temperature were also determined. Thickness homogeneity of the transferred thin Si layer was estimated after SOPL fabrication and after double layer-transfer achievement. The perfect crystallinity of the active Si layer was conserved during the whole process. SOI-like structure fabrication and subsequent separation of thin high quality Si film are reported in order to demonstrate capability of SOPL structure with double-sided layer processing for device integration. [ABSTRACT FROM AUTHOR]

The unique ability of porous silicon (PS) formation-functionalization method to confine organic molecules in a selected layer of a PS stack was demonstrated in this paper. Two different experiments were carried out to prove the confinement of 1-heptyne in a selected layer of a PS stack. The organic molecule was added to the anodizing solution during the formation of the functionalized layer only, and its covalent bonding to the PS surface was assessed by Fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy. At first, a stack with an extremely thin sacrificial layer was prepared in order to separate by brittle fracture the functionalized layer from the unfunctionalized one. Further evidences of spatially selective functionalization were obtained by studying a three-layer sample with the organic molecule confined in the central layer. This will allow developing new applications such as PS multichannel sensors and lab-on-chip systems based on PS. [ABSTRACT FROM AUTHOR]

The electrochemical deposition of poly(para-phenylene)vinylene (PPV) into the pores of porous silicon (PSi) has been investigated using acetonitrile as solvent and p-xylylene-α,α-bis(triphenylphosphonium chloride) as precursor. PPV was successfully deposited onto various Si substrates and PSi layers. It was also deposited inside p+- and p-type PSi. The as-deposited PPV is strongly luminescent. The deposition was confirmed by optical (photoluminescence, reflectivity) and structural (Fourier-transform infrared, scanning electron microscopy) analysis. The deposited PPV was smooth without peeling on PSi layers. With strongly luminescent n+-type PSi, the rate of the deposition was limited by the large resistivity of PSi. The deposition was found dramatically affected by the PSi nanostructure through the diffusion of precursors in the pores and charge carrier transport in PSi. [ABSTRACT FROM AUTHOR]

Porous silicon films were fabricated from p+-Si (0.001-0.005 Ω cm) wafers and characterized for their applicability to wafer bonding and layer transfer schemes. Conditions for producing porous silicon films compatible with both (i) wafer bonding surface smoothness requirements and (ii) layer transfer mechanical requirements were investigated. Nanoindentation of various films showed the quadratic dependence of Young's modulus on relative density. High porosity films (>90%) exhibited prohibitively high surface roughness values for wafer bonding applications, while wafers with lower porosity produced surfaces that were sufficiently smooth; films with midrange Young's modulus were selected for layer transfer applications. Demonstration of wafer bonding of porous silicon surfaces using silicon nitride interlayers was shown without the use of a high temperature anneal step to densify the porous surface or chemical mechanical polishing. Coarsening of the porous structure was observed at temperatures as low as 300°C, and the porous films remained pseudomorphic after annealing at all temperatures, as high as 900°C. Strong bonds were formed at the silicon nitride interfaces after low temperature annealing. After higher temperature annealing, mechanical fracture of the bonded stack occurred in the porous film parallel to the wafer bond interface. A fracture mechanism for these structures is proposed. [ABSTRACT FROM AUTHOR]

We introduced a high porosity ultralow-k film into a Cu damascene interconnect without increasing the dielectric constant (k) of a low-k film after interconnect formation. A high elastic modulus (9 OPa) with ultralow-k value (<21) was achieved by using a self-assembled porous silica film formed with UV irradiation and a silylation anneal. Performing the silylation anneal after trench etching results in recovery of the k value and dielectric properties. A sidewall protection process carried out before metalization protects the low-k film against process-induced damage. The time-dependent-dielectric breakdown lifetime is 10 years at a high electric field of 2.3 MV/cm. [ABSTRACT FROM AUTHOR]

We report on the mechanism of porous silicon (PS) formation from two different fluorohydrogenate ionic liquids. At the first stage of the study, an unexpected formation of PS by the anodic polarization of n-Si in 1-ethyl-3-methylimidazolium (FH)2.3F occurred without illumination. X-ray photoelectron spectroscopy studies show that the PS formation mechanism is based on the Si-F bond formation at the Si/electrolyte interface. At the second stage of the study, the formation of PS from n-Si in Cs(FH)2.3F under anodic polarization was studied without illumination as well. X-ray diffraction studies of the precipitated white film on the silicon wafer during anodic polarization revealed the formation of crystalline Cs2SiF6. A suggested mechanism of PS formation in the ionic liquids is proposed. [ABSTRACT FROM AUTHOR]

We report on a study of the formation of crystalline copper and nanocrystalline cuprous oxide (Cu2O) at electrochemically prepared mesoporous silicon surfaces immersed in a copper sulfate solution. The metal deposition on porous silicon, either by using the reducing properties of the as-prepared native mesoporous surface (immersion plating) or by applying an external current (electroplating), is investigated by IR spectroscopy, X-ray diffraction, and scanning electron microscopy. The degree of oxidation and the surface chemistry of the porous epilayers (porosity 50%, mean pore diameter 10 nm, and pore length 25 μm) are varied by annealing at different temperatures. For immersion plating, we observe the formation of face-centered cubic copper crystals with irregular shapes and a broad size distribution from the nano- to the micrometer scale. Electroplating of as-prepared porous layers leads to a crystalline copper film which homogeneously covers the porous silicon surface. By contrast, electroplating of partially oxidized porous layers leads to isolated single copper crystals with quadratic bipyramidal shape at the outer porous epilayer surface and a sizable fraction of nanochannel confined crystalline Cu2O, a prominent candidate for the investigation and application of quantum confinement effects in semiconductors. [ABSTRACT FROM AUTHOR]

Metal-enhanced HF etching of Si is an electroless method used to produce porous Si. Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide or metal ions. Pd exhibits high activity in enhancing the HF etching of Si without an oxidizing agent even under dissolved-oxygen-free and dark conditions. Electrolessly deposited Pd particles on n-type Si enhance the HF etching of Si but produce no porous layer. Patterned Pd films localize the etching under the boundary of the Pd deposited areas, and thus Pd can produce a microetch pattern on Si with a simple immersion in the HF solution. This etching reaction is explained by electron injection into the conduction band of Si due to the Pd-enhanced anodic oxidization of Si with water and the cathodic hydrogen evolution on Pd with the injected electrons. [ABSTRACT FROM AUTHOR]

Anodic layers are formed on pSi substrates by etching in a HF/MnO2 or a HF/I2 mixed solution. The scanning electron microscopy (SEM) image on the HF/MnO2formed layer reveals that the sample has a doublelayered [highly resistive top/porous Si (PSi)like bottom] structure, while the HF/l2-formed surface provides an SEM image popularly observed on PSi layers formed in a HF/alcohol solution. The HF/MnO2formed sample emits light in the green region but frequently shows a gradual change in the photoluminescence (PL) spectrum from green to red upon exposure to air. The HF/l2formed sample exhibits a stable red emission, which is typically observed in conventional PSi samples. The temperature dependent PL properties of such samples are quite different. The HF/MnO2formed sample can be explained in terms of the usual Arrhenius expression; however, the HF/l2formed sample exhibits a multiplepeak structure at T below ∼60 K, but it disappears at high temperatures. The Xray photoelectron and Fourier transform IR spectroscopies suggest that the highly resistive top layer formed in the HF/MnO2 solution is of any compound of the hydroxyl group or others. The final sample treatment may be a key point for obtaining airstable, greenemitting layers in the HF/MnO2 solution. [ABSTRACT FROM AUTHOR]

A comprehensive study has been performed on the light-emitting properties of porous silicon (PSi) films formed by anodic etching. PSi films are investigated using photoluminescence (PL), PL excitation, and X-ray photoelectron spectroscopy (XPS). The PL emission features are strongly dependent on the anodic current density. Analysis of the PL spectrum suggests the existence of two different emission mechanisms in the PSi layers: An efficient mechanism that is usually observed at higher temperatures and one that exhibits a multiple-peak Structure and appears only at low temperatures (T ⩽ 60 K), even at an excitation energy (∼3.8 eV) far from the PL resonance band (1.5-2.4 eV). The high temperature emission can be interpreted as being due to the quantum-confinement effect, i.e., relaxation of the momentum conservation at and above the indirect absorption edge (supra-EIDg emission), while the lower temperature PL emission is explained by the classical configurational-coordinate model with a lattice vibronic quantum of ∼64 meV. The XPS data indicate that the short-range order is important in understanding the optical spectra not only for the amorphous substance but also for the nanocrystalline substance. [ABSTRACT FROM AUTHOR]

We present a technique involving the use of pulsed anodization for porous silicon (PS) thin-film fabrication on low doped substrates, for which the interface roughness and porosity gradients usually observed in such films can be eliminated. The work presented includes a detailed characterization of the effects of duty cycle and frequency during pulsed anodization. The study spans pulsing frequencies of 0.1-1000 Hz and duty cycles of 5-50%. The combination of low frequency (0.1 Hz) and low duty cycle (5%) for the pulse train used for anodization produces PS thin films, displaying no measurable interface roughness or porosity gradient. The mechanisms behind the wide variation in available PS thin-film properties with pulsed anodization parameters are analyzed using a galvanostatic technique. The analysis indicates that the inhomogeneity and roughness observed in PS films fabricated on low doped starting wafers are both due to the unstable multistep dissolution kinetics of silicon during PS film formation. [ABSTRACT FROM AUTHOR]

ZnS films were prepared by pulsed laser deposition (PLD) on porous silicon (PS) substrates. This paper investigates the effect of annealing temperature on the structural, morphological, optical and electrical properties of ZnS/PS composites by x-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) and I-V characteristics. It is found that the ZnS films deposited on PS substrates were grown in preferred orientation along b-ZnS (111) direction, and the intensity of diffraction peak increases with increasing annealing temperature, which is attributed to the grain growth and the enhancement of crystallinity of ZnS films. The smooth and uniform surface of the as-prepared ZnS/PS composite becomes rougher through annealing treatment, which is related to grain growth at the higher annealing temperature. With the increase of annealing temperature, the intensity of self-activated luminescence of ZnS increases, while the luminescence intensity of PS decreases, and a new green emission located around 550 nm appeared in the PL spectra of ZnS/PS composites which is ascribed to the defect-center luminescence of ZnS. The I-V characteristics of ZnS/PS heterojunctions exhibited rectifying behavior, and the forward current increases with increasing annealing temperature. [ABSTRACT FROM AUTHOR]

In this paper, a complementary metal-oxide semiconductor (CMOS)-compatible silicon substrate optimization technique is proposed to achieve effective isolation. The selective growth of porous silicon is used to effectively suppress the substrate crosstalk. The isolation structures are fabricated in standard CMOS process and then this post-CMOS substrate optimization technique is carried out to greatly improve the performances of crosstalk isolation. Three-dimensional electro-magnetic simulation is implemented to verify the obvious effect of our substrate optimization technique. The morphologies and growth condition of porous silicon fabricated have been investigated in detail. Furthermore, a thick selectively grown porous silicon (SGPS) trench for crosstalk isolation has been formed and about 20dB improvement in substrate isolation is achieved. These results demonstrate that our post-CMOS SGPS technique is very promising for RF IC applications. [ABSTRACT FROM AUTHOR]

Porous silicon (PS) with a micro-nano-hybrid structure has been successfully fabricated with an electrochemical etching process. The micropores consist of one-dimensional tunnels, which vary from ca. 1 to 1.5 µm in pore diameter and extend up to 15 µm in depth. The walls of these micropores are covered with a nanoporous structure that consists of small spherical particles, the feature size of which is of the order of tens of nanometers or smaller. The as-prepared PS structures show cathodic/anodic peaks for lithiation/delithiation during cyclic voltammetry with minimal destruction of either the micropore or nanopore of its wall even after 50 cycles. Furthermore, the peak current, the cumulative charge increase, and the electrochemical impedance for electrode reactions consistently decreases with the surface area of the tunnel wall, indicating that processes at the tunnel wall govern the overall lithiation/delithiation reactions. A hybrid porous structure consisting of microtunnels with nanostructured surface layers appears to provide a viable and practical way to utilize silicon for anode materials in rechargeable microbatteries. [ABSTRACT FROM AUTHOR]

In this study, the interface chemistry and adhesion strength between a porous SIOCH extra-low-dielectric-constant film and SiCN etch stop layers were investigated with different plasma treatments. An interlayer of -6 nm thick between the porous SiOCH film and SiCN layers was found to be composed of Si, N, C, and 0. The SiOCH/SiCN interface was constructed by mixing bonds, including Si-C-N, Si-N-C, Si-O-C, Si-O2, etc. Under H2 and NH3 plasma treatments, a large amount of weak Si-CH3 and SiO-CH3 bonds were broken, and more Si-O related bonds of high binding energy formed at the interfaces. Moreover, under the accumulation of sufficient shear stresses around the indented regions during nanoindentation tests, interface delamination between the porous SiOCH film and SiCN layers occurred. The interface adhesion energy between untreated porous SiOCH film and SiCN layers was accordingly measured as 1.68 J/m2. After plasma treatments, especially NH3 plasma, the adhesion strength was effectively improved to 2.13 J/m2. [ABSTRACT FROM AUTHOR]

With a goal of preparing silicon-based interactive filters in the micropore range, we have developed the first single-step etch-liftoff procedure (one-step separation) based on the formation and removal of macroporous silicon layers. With silicon wafers whose resistivities are in the range from 14 to 22 Ω cm, we are able to create, in a surprisingly controlled manner, films whose pore diameters range from 1-2 μm and whose thickness ranges from 3 to 70 μm. These silicon-based films (filters), which carry a polarizing negative charge, represent an alternative to porous alumina (films) filters (pore diameter not yet in the 1-2 μm range) both in terms of their size range and potential interaction-reaction at elevated temperature. Preliminary experiments demonstrate that platinum and copper can be introduced to these filters using electroless solutions with a goal to creating an effective reductive surface. Using these and alternate material combinations, interactive-reactive filters in the 1 μm size range that operate at temperatures well in excess of porous polymer films can be realized. [ABSTRACT FROM AUTHOR]

In this work, we describe the synthesis of porous Au structures by galvanic displacement reaction using porous Si exhibiting arrays of parallel pores with diameters near 100 nm as template material. Two metal ions were used as Au precursors: tetrachloroaurate(III) and bis(ethylenediamine)Au(III). Fluoride was employed to etch the Si oxides that form as a result of the displacement reaction. The porous metal layer that forms with tetrachloroaurate(III) is an agglomerate of spherical particles with Ca. 100 nm diameter. Metallization with bis(ethylenediamine)Au(III) preserves the morphology of the original porous Si layer, Creating a porous Au layer containing an array of parallel pores with Ca. 100 nm diameter. The displacement process is under diffusion control for both reagents. The expansion rate of the porous metal layer into the substrate agrees well with a simple one- dimensional reaction-diffusion model entailing the displacement of Si in a large extent. The electrochemical behavior of the synthesized porous Au layers was studied by voltammetry. [ABSTRACT FROM AUTHOR]

Porous silicon samples were obtained by the electrochemical method and were impregnated with Pd by the electroless process. X-ray photoelectron spectroscopy illustrated that the surface of the samples is oxidized during the palladium deposition. Scanning electron microscopy showed how factors such as morphology and pre-oxidations of porous samples and the plating parameters including Pd-salt concentration, HCl concentration, temperature of the electroless solution and the time of process affect the growth and nucleation of Pd particles. Observations demonstrated that the illumination-assisted process on p-type samples has a drastic effect on the growth of palladium. Hydrogen sensing of these samples, which works on the basis of change in the Schottky barrier between the silicon and the palladium interface, was tested. Variations of the electrical resistance in the presence of diluted hydrogen at room temperature revealed that the best samples can sense hydrogen at levels down to several thousand ppm. This value is far below the flammability limit of hydrogen gas. In addition, the best samples have a very good selectivity to hydrogen. [ABSTRACT FROM AUTHOR]

1-Dodecyne organic molecules (OMs) were chemically bonded to porous silicon (PS) to constitute Si-PS-OMs-metal (M) junctions. PS, which is highly susceptible to atmospheric contamination, becomes stable upon chemical passivation of its surface atoms. Current-voltage curves were studied to understand the charge transport properties, and these measurements indicate clearly that organic passivation unpins the Fermi-level at the PS-M interface. It was believed earlier that Si-PS is the only important interface that controls charge transport in these junctions. We observe that both PS-M and Si-PS interfaces are equally important in influencing the junction parameters when surface states are passivated. The results are discussed in terms of energy band diagrams of these junctions. [ABSTRACT FROM AUTHOR]

A scanning thermal microscope (SThM) in the dc regime was used to study the thermal conductivity of homogeneous in-depth meso-porous silicon in the form of thin films on a monocrystalline silicon substrate. Measurements for different film porosities (30-80%) and thicknesses (100 nm-8 µm) were performed in order to estimate the influence of both layer porosity and thickness on the thermal conductivity values of porous silicon (PS). An analytical model predicting the SThM measurement in the case of ultra-thin monolayered samples was used to calibrate the technique, to analyse experimental data and to determine the thermal conductivity of meso-porous layers. Effective thermal conductivity of meso-PS films was found to decrease when the porosity increases. The effective thermal conductivities measured for thick porous layers (several µm) are in good accordance with those measured by micro-Raman-spectroscopy on bulk meso-PS samples. For submicrometric thicknesses (<1 µm), the effective thermal conductivity of layers decreases significantly with decreasing layer thickness due to the increased sensitivity of measurements to the thermal resistance of the film/substrate interface. An intrinsic thermal conductivity of PS was calculated independently of the film thickness and the values of interfacial thermal resistances were thus estimated. From the apparatus point of view, the results obtained show that the depth being sensed is of the order of a few micrometres for insulating materials and depends on the thermal conductivity of the films. [ABSTRACT FROM AUTHOR]