Enhancement and stabilization of the photoluminescence from porous silicon prepared by Ag‐assisted electrochemical etching

In this paper, we present the results of studies on the photoluminescence (PL) of porous silicon (PSi) samples obtained by etching with the assistance of silver metal in different ways. If the Si sample, after being coated with a layer of silver nanoparticles, is electrochemically etched, its PL intensity becomes hundreds of times stronger than the PL intensity when it is chemically etched in the similar conditions. The difference in the PL intensities is explained partly by the anodic oxidation of silicon which occurs during the electrochemical etching process. The most obvious evidence that silicon had been oxidized anodically in the electrochemical etching process is the disappearance of the PSi layer and the appearance of the silicon oxide layer with mosaic structure when the anodization current density is large enough. The anodic oxidation has the effect of PSi surface passivation. Because of that, the PL of obtained PSi becomes stronger and more stable with time. Copyright © 2012 John Wiley & Sons, Ltd.     

Electrochemical etching of silicon carbide

Both n- and p-type SiC of different doping levels were electrochemically etched by HF. The etch rate (up to 1.5 μm/min) and the surface morphology of p-type 6H-SiC were sensitive to the applied voltage and the HF concentration. The electrochemical valence of 6.3 ± 0.5 elementary charge per SiC molecule was determined. At p-n junctions (p-type layer on a n-type 6H-SiC substrate) a selective etching of the p-type epilayer could be achieved. For a planar 6H-4H polytype junction (n-type, both polytypes with equal doping concentrations) the 4H region was selectively etched under UV illumination. Thus polytype junctions could be marked by electrochemical etching. With HCl instead of HF no etching of SiC occurs, but a SiO₂ layer (thickness up to 8 μm) is formed by anodic oxidation. Received: 29 October 1998 / Accepted: 27 January 1999     

Hybrid plasma bonding for void-free strong bonded interface of silicon/glass at 200 °C

A novel hybrid plasma bonding (HPB) that combines sequential plasma activation (reactive ion etching followed by microwave radicals) with anodic bonding has been developed to achieve void-free and strong silicon/glass bonding at low temperature. The interfacial voids were observed at the silicon/glass interface both in the anodic bonding and in the plasma activated anodic bonding, but the voids were completely disappeared in the HPB method at 200 °C. The bonding strength of the silicon/glass in the HPB was as high as 30 MPa at 200 °C, which was higher than that in the individual treatment of anodic and plasma activated bonding methods. The improved characteristic behavior of the interface in the HPB is attributed to the higher hydrophilicity and smooth surfaces of silicon and glass after sequential plasma activation. These highly reactive and clean surfaces enhance the mobility of alkaline cations from the glass surface across the interface toward the bulk of glass in the HPB. This transportation resulted in a ∼353 nm thick alkaline depletion layer in the glass and enlarged the amorphous SiO₂ across the interface. The void-free strong bonding is attributed to the clean hydrophilic surfaces and the amorphous SiO₂ layer across the interface.     

Influence of the electrolyte composition and pH on the anodic dissolution of p-type Si in aqueous HF solutions

The influence of the electrolyte composition and pH on the anodic currents obtained during electrochemical etching of p-type silicon in aqueous HF solutions has been investigated. Original and accurate pH measurements were performed to characterize the exact composition of the HF + H₂O electrolytes commonly used. It is shown that for these very acid solutions (pH < 2) almost all fluoride is in the form of the non-dissociated HF species which appears to play a preponderant role in the silicon dissolution reaction kinetics. The effect of pH can be restricted to its influence on the modification of the different concentrations by shifting the equilibria.     

电化学微/纳加工分辨率的影响因素及对策

The etching resolution of electrochemical fabrication technique is influenced significantly by the diffusion layer of the etchant. It has been shown that a fast etching rate can achieve higher etching resolution due to so-called heterogeneous scavenging effect, while a lower etching rate will result in rather lower etching resolution. For the latter case, the confined etchant layer technique(CELT) has been employed to improve the etching resolution. i. e., a certain redox couple which can consume the etchant homogeneously and rapidly was added to the solution. The homogeneous scavenging effect confined the etchant within a narrow layer around the electrode surface and much improved etching resolution was achieved. Using the CELT and a needle-shaped microelectrode, an etching spot of several micro-meters was obtained at silicon wafer surface.     

Electrochemical oxidation of a carbon black loaded polymer electrode in aqueous electrolytes

The suitability of a polymeric composite material for use as part of an anode structure in a cathodic protection system has been examined. The composite material was a conductive blend (volume resistivity typically 1.5 Ω cm) of carbon black in a polyethylene binder. A long operational lifetime for the material demands that the rate of carbon loss must be low. In the work reported here, electrochemical and in situ analytical techniques were employed to characterise the performance of the material over a wide range of anodic current densities in a variety of aqueous electrolytes. The predominant anodic electrochemical reaction on the polymeric material is CO₂ formation in acid and neutral solutions, which causes loss of carbon from the surface and the development of a non-conducting layer of polyethylene. The characteristics of the reaction suggest that it occurs via the discharge of H₂O. In alkaline pH, however, the anodic reactions are more complex. A high OH⁻ concentration (pH 12 or higher) favours the formation of oxygen rather than CO₂, particularly at low anodic potentials. The presence of CO₃ ²⁻ in the electrolyte catalyses the evolution of oxygen at pH values as low as 9. The electrochemical formation of oxygen always occurs in parallel with the generation of some humic acid in the solution. Received: 23 July 1998 / Accepted: 17 November 1998     

Investigation of the incorporation of electrolyte anions in porous anodic Al2O3 films by employing a suitable probe catalytic reaction

A new method has been developed capable of describing the incorporation of electrolyte anions along the pore wall surface and across both the barrier layer and the pore wall oxide after the establishment of the steady state of growth of porous anodic Al₂O₃ where other methods cannot be applied to obtain reliable results. The knowledge of the nature/composition of anodic oxides as regards the incorporation of species like electrolyte anions is of specific importance for both the understanding of the electrochemical mechanism of oxide production and growth and the scientific and technological applications of porous anodic Al₂O₃ films. The method consists of the selection and use of a suitable catalytic probe reaction on porous anodic oxides at thicknesses varying from a value near zero up to the maximum limiting thickness and the treatment of the experimental reaction rate results by a properly developed mathematical formalism. This method was employed in anodic Al₂O₃ films prepared in H₂SO₄ anodizing electrolyte at a constant bath temperature and different current densities using as a probe reaction the decomposition of HCOOH on these oxides, which is almost exclusively a dehydration reaction, at relatively high reaction temperatures, 350 °C and 390 °C, where the effect of other species except SO₄ ²⁻ incorporated in the oxide on the reaction rate is eliminated. It has been shown that the fraction of the intercrystallite surfaces occupied by SO₄ ²⁻ follows a parabola-like distribution. It has a significant value at the pore base surface, depending on the current density, then it passes through a maximum along the pore wall surface and across both the barrier layer and the pore walls near the pore bases at positions depending on the current density and then becomes almost zero at the mouths of the pores of the oxide with the maximum limiting thickness and at both the Al₂O₃/Al interface and cell boundaries. The maximum value of the surface coverage is almost independent of the current density and is always near 1, showing an almost complete saturation of intercrystalline surfaces at these positions. The above distribution of surface coverage predicts a qualitatively similar distribution of the SO₄ ²⁻ bulk concentration across both the barrier layer and pore wall oxide around the pore bases. The method may be improved and developed further either for a more detailed investigation of the above films or to investigate films prepared in other pore-forming electrolytes. Received: 30 July 1998 / Accepted: 30 September 1998     

碱性水溶液中甲醛在硅电极表面的电化学行为及其对硅化学刻蚀的影响

研究了KOH水溶液中氧化剂甲醛在p-Si和n-Si(100)单晶半导体电极表面的电化学行为及其对硅化学刻蚀表面形貌的影响．实验结果表明,甲醛不仅影响p-和n-型半导体电极在碱性溶液中的阳极氧化峰电流,而且在负电位区能在Si(100)电极上发生还原．在光照条件下,p-Si(100)电极上也观测到了HCHO的电化学还原及光电流倍增效应．甲醛在硅电极表面的电化学还原反应分两步进行,反应终产物为甲醇．此外,HCHO能有效抑制碱性溶液中Si表面“金字塔”型表面粗糙颗粒的形成。     

发光多孔硅的光谱及电化学研究

单晶硅是E_g为1.1eV的间接带隙半导体材料,在可见光区不发光,不能应用于光电子领域.但是,Canham 1990年首次发现^[1],适当条件下形成的多孔硅在室温下就可发出强度能与Ⅲ-Ⅴ族半导体发光二极管相媲美的可见光。     

Electrochemical behaviour of p-Si in methanol solutions of chlorides

The mechanism of anodic dissolutions of p-Si single crystals in CH₃OH–LiCl and CH₃OH–LiCl–HCl solutions was investigated by means of the following electrochemical methods: linear sweep voltammetry, the potentiostatic transient technique and XPS surface analysis. The dissolution of p-Si proceeds by a two-step mechanism with the creation of a Si(II) surface intermediate. At low anodic overvoltage the dissolution proceeds with the formation of porous silicon, probably through the reaction: 2Si(II)Si+Si(IV). Structural etching of the single crystals surface was observed at high anodic overvoltage (E>2 V). At this potential range, silicon dissolves with the formation of a Si(IV) soluble product. Electrolysis of the methanol solvent containing Si(IV) in the cell p-Si|CH₃OH–LiCl–Si(IV)|M, where M=Pt, Cu or 18/8 stainless steel, leads to the deposition of an amorphous organosilicon layer on the cathode. The analysis of the deposit performed by means of XPS, FTIR and SEM allows determination of the morphology and composition of the film. The layer consists of Si–OCH₃ compounds and can be created only in methanol solvent. The film is unstable in a humid atmosphere and undergoes transformation into a Si–OH layer.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdansk-Sobieszewo, Poland, 23–26 April 2003Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

Study on Microcontamination of Silver onto p-Type Crystalline Silicon(111) from an Aqueous Solution Using Cyclic Voltammetry

The microcontamination process of silver onto p-type crystalline silicon(111) in a solution of 0.01 mol L⁻¹ AgNO₃ at room temperature was investigated by studying the anodic stripping behavior using cyclic voltammetry (CV). This paper shows that the rate of Ag deposition is rapid and that deposition is almost fully accomplished within 1 s. Calculating the surface coverage (Γ) for 1 s, 10 min, or 1 h immersion based on the CV curves demonstrated that the silver layer was only a monolayer.     

Investigation of the anodic decomposition of the solid electrolyte RbCu4Cl3I2

The process of electrochemical decomposition of the solid electrolyte RbCu₄Cl₃I₂ at a vitreous carbon electrode has been investigated. The anodic decomposition of the electrolyte occurs in two steps. At first, the oxidizing electrode reaction of Cu⁺ ions to Cu²⁺ ions takes place at potentials higher than 0.57 V and a layer of decomposition products is formed on the electrode surface, including the divalent copper compound RbCuCl₃. Then the oxidizing reaction of I^– ions occurs at potentials higher than approximately 0.67 V, with deposition of an iodine layer onto the electrode surface. The deposition rate of the layers of decomposition products is controlled by instantaneous nucleation and two-dimensional growth of the deposit. The total thickness of the passivating layer of decomposition products on the anode is equal to ca. 1 μm. Electronic Publication     

Differential thermal analysis of porous silicon

Porous silicon materials, macro- and mesoporous silicon, obtained by electrochemical anodic etching of n- and p-Si were studied by differential thermal analysis at a steady temperature rise and under isothermal conditions in nitrogen atmosphere and in air. The method was used to estimate the presence and amount of phases of surface volatile compounds. The possibility was studied to perform a comparative estimate of the specific surface area of different types of porous silicon from data on the surface oxidation kinetics determined by the dynamic differential thermal analysis in air.     

Low-voltage anodized TiO<Subscript>2</Subscript> nanostructures studied by alternate current electrochemical microscopy and photoelectrochemical measurements

This paper presents the characterization of TiO₂ nanostructures obtained by low-voltage anodization using alternate current electrochemical microscopy (AC-SECM) and photoelectrochemical (PEC) measurements. TiO₂ nanostructures were obtained from the exposure of titanium foils to several aqueous acidic solutions of hydrofluoric acid + phosphoric acid at potentials of 1 to 3 V. Scanning electron microscopy, X ray diffraction, and atomic force microscopy studies evidence the formation of a thin porous amorphous layer (<600 nm) with pore size in the range of 200–1,000 nm. By AC-SECM studies at different bias, we were able to confirm the unambiguous semiconducting properties of as-obtained porous titania films, as well as differences in surface roughness and conductivity in specimens obtained at both potentials. The difference in conductivity persists in air annealed samples, as demonstrated by electrochemical impedance spectroscopy and PEC measurements. Specimens obtained at 3 V show lower photocurrent and dark current than those obtained at 1 V, regardless of their larger conductivity, and we proposed it is due to differences on the oxide layer formed at the pore bottom.     

Determination of phosphorus distribution in the region of a SiO2−Si interface by substoichiometric analysis

A simplified method for the substoichiometric analysis of phosphorus has been developed and applied to determine the concentration distribution of phosphorus in the region of a SiO₂–Si interface in order to explain why phosphorus is lost from the ion-implanted silicon surface throughout the oxidation and oxide removal processes. It is revealed that phosphorus piles up on the SiO₂ side at the interface by the thermal oxidation of silicon surface and is removed with the oxide by wet etching and with the resulting silicon by RCA cleaning. This results in a total loss of ion-implanted phosphorus of 3.5%.     

Relationships between microstructure and pitting corrosion of ADI in sodium chloride solution

Austempered ductile iron (ADI) has complex microstructure containing a multiphase matrix (called ‘ausferrite’), graphite spheres and oxide inclusions. The corrosion resistance of ADI is related to its microstructure which is determined by heat treatment parameters (like austempering temperature, austempering time, austenitising temperature and austenitising time). In the present paper, the electrochemical behaviour and corrosion resistance of ADI have been investigated by means of the electrochemical microcell technique and classical electrochemical measurements in sodium chloride solution. Particular attention has been paid to the influence of austempering temperature on the microstructure and pitting corrosion. It has been shown that ADI austempered at 430 °C has upper ausferritic microstructure and reveals a better corrosion resistance in sodium chloride solution than ADI austempered at 280 °C. Moreover, the corrosion resistance increases as the volume fracture of ferrite increases and the carbon content of austenite decreases. The good corrosion behaviour of ADI austempered at 430 °C was also related to the good coarsening of the austenite grains and broad ferrite needles (less ferrite/austenite interfaces). It has been demonstrated that silicon is the alloying element hindering the anodic dissolution of the alloy.     

Electrochemical behavior of passive films on Al–17Si–14Mg (wt.%) alloy in near-neutral solutions

The anodic polarization behavior of alloy Al–17Si–14Mg in borate solutions with and without 0.01 M NaCl was compared to that for pure Al. Results showed that, for the alloy, the passive current density increased but the pitting susceptibility decreased. The first effect was ascribed to a significant electrochemical activity of the Mg₂Si intermetallics and the second to improved stability of the oxide film. X-ray photoelectron spectroscopy analysis of potentiostatically formed passive film on the alloy showed that it consisted of aluminum oxyhydroxide with incorporation of silicon in its elemental and two oxidized states (+3 and +4). Mott–Schottky analysis showed that trivalent silicon ion acted as an n-type dopant in the film. The interrelationship between passive film composition, electronic properties, and pitting behavior has been discussed.     

Study of the Electroless Silver Seed Formation on Silicon Surface

The silver seed on silicon was prepared through aqueous I-IF and AgNO3 solution at room temperature. In order to explore the formation process of silver seed on silicon, the methods of open circuit potential with time （OCP-t）, anodic stripping sweep voltammetry （ASV） and scanning tunneling microscope （STM） were used in this work. The procedure of silver nucleus growing into large particles was explained by electro-negativity. The growth mechanism of silver seed on silicon has been presented： at first, the silver monolayer and multilayer firstly grows onto silicon without fully covering the surface at the expense of silicon etching due to the silver seed attracting the electron from silicon, after that, the monolayer coalesces together, forming continuous grain film with some silver atoms diffusing into the silicon and the multilayer still grows thick simultaneously.     

Preparation and properties of mechanically alloyed and electrochemically etched porous Ti–6Al–4V

Formation of porous Ti–6Al–4V nanostructure biomaterial was described. The alloy was prepared by mechanical alloying followed by pressing, sintering and subsequent anodic electrochemical etching in 1 M H₃PO₄ + 2% HF electrolyte at 10 V for 30 min. Mechanically alloyed Ti–6Al–4V has nanostructure with grain size of about 35 nm and large grain boundaries volume fraction, which essentially improve etching process. The electrolyte penetrates sintered compacts through the grain boundaries, resulting in effective material removing and pores formation. The pore diameter reaches up to 60 μm, which is very attractive for strong bonding with bone. The anodization of the microcrystalline alloy ingot results in selective etching, revealing of the two-phase structure with relatively flat surface. The corrosion properties were investigated in Ringer’s solution. Mechanically alloyed samples shows worse corrosion resistance than the bulk microcrystalline alloy ingot, but electrochemical etching results in improving corrosion resistance.     

The effect of the type of conductivity on the initiation and formation of pores in silicon during electrochemical etching

The effect of the type of Si conductivity on the initiation and formation of pores in silicon samples during electrochemical etching was studied. The difference between the pore formation processes in n- and p-conducting silicon was attributed to the properties and the nature of layers formed in the initial period of etching on the Si surface. A possible composition of the layers formed on the Si surface was proposed considering the chemistry of Si interaction with the etching agent.