The dependence of hydrofluoric acid concentration during anodization on photoluminescence of porous silicon

The mixture of hydrofluoric (HF) acid and ethanol is used as an electrolyte during anodization of silicon. We investigated the effect of the ratio of HF acid to ethanol on photoluminescence. It is concluded that porous silicon anodized with the electrolyte containing 35 or 40% HF acid provides strong photoluminescence. The fact implies the existence of a chemical reaction including ethanol during anodization other than electrochemical reaction.     

近规整多孔硅电化学制备与表征

电化学阳极氧化条件对多孔硅孔排列的规整度有着显著的影响.提出了一种不需阳极氧化铝模板或预图案化而直接制备近规整多孔硅的电化学方法,分析了氧化时间、电解液组成、HF浓度对多孔硅形态的影响.结果表明,随着阳极氧化时间的增加,多孔硅孔的深度逐渐加大,孔径则呈先增大后稳定的趋势.当氢氟酸(40%)与N-N-二甲基甲酰胺(DMF...     

Effect of ultraviolet illumination and ambient gases on the photoluminescence and electrical properties of nanoporous silicon layer for organic vapor sensor

The purpose of this research, the nanoporous silicon layer were fabricated and investigated the physical properties such as photoluminescence and the electrical properties in order to develop organic vapor sensor by using nanoporous silicon. The Changes in the photoluminescence intensity of nanoporous silicon samples are studied during ultraviolet illumination in various ambient gases such as nitrogen, oxigen and vacuum. In this paper, the nanoporous silicon layer was used as organic vapor adsorption and sensing element. The advantage of this device are simple process compatible in silicon technology and usable in room temperature. The structure of this device consists of nanoporous silicon layer which is formed by anodization of silicon wafer in hydrofluoric acid solution and aluminum electrode which deposited on the top of nanoporous silicon layer by evaporator. The nanoporous silicon sensors were placed in a gas chamber with various organic vapor such as ethanol, methanol and isopropyl alcohol. From studying on electrical characteristics of this device, it is found that the nanoporous silicon layer can detect the different organic vapor. Therefore, the nanoporous silicon is important material for organic vapor sensor and it can develop to other applications about gas sensors in the future.     

A technique for the fabrication of various multiparametric porous silicon samples on a single substrate

Porous silicon (PSi) samples generally have a uniform thickness and pore size according to specific anodization conditions, as the Si wafer is entirely immersed into hydrofluoric (HF) acid during the anodization process. In contrast, multiparametric (MP) PSi, as described in the present work, is fabricated by inserting a Si wafer gradually (or by stages) into a HF solution during the anodization process. Consequently, MP-PSi allows single layer fabrication with a pore-size and layer thickness gradient or various multilayers, on a single substrate. Therefore, MP-PSi can be readily used in sensor application areas to determine optimized detection conditions for various materials, such as gas, liquid, and bio-materials. MP-PSi layer with a lateral pore gradient distribution can also be used as size-exclusion matrix. In addition, the MP-PSi multilayer array is expected to open up application areas involving optical electronic nose systems.     

Nanoporous ZnO thin films deposited by electrochemical anodization: effect of UV light

Nanoporous ZnO thin films were deposited by electrochemical anodization of high purity Zn at room temperature using Pt counter electrode, calomel reference electrode and oxalic acid as the electrolyte. The crystallinity and the surface morphology were studied by X-ray diffraction (XRD) and Field emission scanning electron microscope (FESEM). The variation in molar concentration of oxalic acid during anodization had significant effect on the crystal size and the pore size particularly in the presence of UV light. An increase in room temperature band gap from 3.25 to 3.87 eV of ZnO film grown in 0.3 M oxalic acid indicates a quantum confinement effect and it was further confirmed by a blue shift of the photoluminescence (PL) spectra. A possible mechanism of the anodization and the photoetching in the presence of UV light of the ZnO film have been suggested.     

Anodic oxidation as sectioning technique for the analysis of impurity concentration profiles in silicon

Anodic oxidation is a commonly used technique for impurity profile determination in silicon. Optimum electrolyte composition, anodization conditions and problems involved in the determination of silicon removed by anodization are examined together with effects due to sample preparation and silicon characteristics. Experimental data are presented for the use of the ethylene glycol electrolyte together with the limits of accuracy of the technique. The use of the ellipsometric method in the evaluation of refractive index and thickness of silicon anodic oxides is illustrated and discussed.     

Corrosion protection of aluminium-matrix aluminium nitride and silicon carbide composites by anodization

Anodization is an effective surface treatment for improving the corrosion resistance of aluminium-matrix composites. For SiC particle-filled aluminium, anodization was performed successfully in an acid electrolyte, as usual. However, for AlN particle-filled aluminium, anodization needed to be performed in an akaline (0.7 N NaOH) electrolyte instead of an acid electrolyte, because NaOH reduced the reaction between AlN and water, whereas an acid enhanced this reaction. The concentration of NaOH in the electrolyte was critical; too high a concentration of NaOH caused the dissolution of the anodizing product (Al2O3) by the NaOH, whereas too low a concentration of NaOH did not provide sufficient ions for the electrochemical process. The corrosion properties and anodization characteristic of pure aluminium, Al/AlN and Al/SiC were compared. Without anodization, pure aluminium had better corrosion resistance than the composites and Al/SiC had better corrosion resistance than Al/AlN. After anodization, the corrosion resistance of Al/AlN was better than Al/SiC and both composites were better than pure aluminium without anodization, but still not as good as the anodized pure aluminium.     

Determination of Vickers microhardness on porous silicon surfaces

The Vickers microhardness values of two different sets of porous silicon layers were determined at applied load of 98 mN. The sets consisted of Boron-doped substrates anodized at diverse current densities for two different amounts of hydrofluoric acid (HF) in the etching solution. We found that the microhardness of the samples with lower content of HF at the anodization process showed higher values, whereas the Vickers parameter diminishes consistently for higher current densities. A possible explanation of this behavior is proposed.     

Magnesium outdiffusion from porous silicon carbide substrates during autodoping of gallium nitride epilayers

We have studied the outdiffusion of magnesium from silicon carbide substrates during autodoping of gallium nitride epilayers. The autodoping effect was observed in the case of porous substrates obtained by surface anodization of 6H-SiC wafers. It is established that the magnesium distribution profiles can be controlled by post-growth annealing. The fact of doping is confirmed by the results of photoluminescence measurements at 77 K.     

Characterization of the anomalous luminescence properties from self-ordered porous anodic alumina with oxalic acid electrolytes

The pore height and diameter of the nanoscale structure of porous anodic alumina (PAA) film produced by the anodization technique are controllable. The structures can be applied for the fabrication of visible spectral range optical devices. In this study we characterized the luminescence properties of self-ordered PAA films evaporated onto silicon substrates. Anomalous luminescence properties produced by carrier confinement were observed in PAA films fabricated with the introduction of oxalic acid electrolytes during the anodization process. The recombination mechanisms were characterized by measuring the temperature-dependent photoluminescence (PL) spectra. The PL spectra of PAA films show an asymmetrical luminescence profile in the blue emission region. The Gaussian function divides these into two subbands. The subbands originate from two different kinds of oxygen-deficient defect centers, namely, F⁺ (oxygen vacancy with only one electron) and F (oxygen vacancy with two electrons) centers. The F centers are densest at the surface but show a gradual decrease with an increase in the pore wall depth and electrolyte concentration. However, the reverse trend is observed for the F⁺ centers. In strong contrast to the commonly expected trend of a uniform reduction in non-radiative recombination with decreasing lattice temperature, we observed an anomalous low-temperature PL growth and decline between the F and F⁺ centers. Theoretical models corroborate the anomalous temperature behavior. All the calculations are in agreement with the experimental observations.     

The study of highly crystalline ZnSe growth on porous silicon

The objective of this study relates to producing a ZnSe epilayer on porous silicon. In the subsequent process, ZnSe having a direct energy gap of 2.68 eV at room temperature was grown on porous silicon under differing anodization conditions utilizing chemical vapor deposition techniques, resulting in the successful growth of a single crystal ZnSe epilayer on the porous silicon substrate. The characteristics of the epilayer were then analyzed by X-ray diffraction and photoluminescence. The photoluminescence spectrum of the single ZnSe epilayer exhibited good emission properties at 444 nm (2.792 eV), 457 nm (2.718 eV), 478 nm (2.594 eV), and 574 nm (2.16 eV).     

Effect of annealing in various atmospheres on the photoluminescence of porous silicon

The effect of isochronous annealing in various atmospheres on the photoluminescence of porous p-type silicon samples obtained by anodization under identical conditions was studied. Comparative experimental data for the samples annealed in air, nitrogen, and vacuum are presented.     

Argon-oxygen ion-plasma treatment modifies the surface composition and photoluminescence spectrum of porous silicon

We have studied changes in the surface composition and photoluminescence spectrum of porous silicon (por-Si) during the ion-plasma etching of samples in an argon-oxygen gas mixture. This treatment leads to the passivation of the surface of quantum filaments by residual fluorine and the formation of silicon oxide. The source of fluorine atoms are HF molecules retained in the volume of pores upon the por-Si structure formation by chemical etching. Increase in the fluorine concentration is accompanied by the growth in intensity of the blue-green band and broadening of the red band in the photoluminescence spectrum of por-Si.     

Mechanisms of formation and topological analysis of porous silicon — computational modeling

Porous silicon formation under anodization in HF solution is studied by means of computer simulation, using the model, which takes into account the Si dissolution, thermal generation, diffusion and drift of holes and quantum confinement. Structures of porous layer, obtained in computational experiments at various doping level of initial Si, temperatures, HF concentrations, anode current densities, are shown and analyzed. The porosity and fractal dimension of the obtained porous structures are also analyzed.     

Morphology of porous silicon structures formed by anodization of heavily and lightly doped silicon

By computer simulation the growing process of porous silicon under p⁻-Si and p⁺-Si anodization in HF solution is studied. The model of electrochemical etching of p⁺-Si includes the relief selective mechanism, which allows one to establish the relationship between anodization conditions (current density, HF concentration, temperature and doping level) and the topological characteristics of porous silicon (PS). The simulation of p⁻-Si dissolution is based on the model of diffusion limited aggregation (DLA), taking into account the thermal generation of holes and the quantum confinement effect. The various morphology of simulated PS structures exhibits a close resemblance to that of experimental ones formed in p⁺-Si and p⁻-Si wafers. For simulated p⁻-Si-based PS layers the porosity profiles and fractal dimension are calculated. It is shown that PS in p⁻-Si is multifractal with fractal dimension varying monotonously from 0.1 to 3 with size increase.     

Fourier transform infrared detection in miniaturized total analysis systems for sucrose analysis

In this work, a flow system containing a micromachined lamella-type porous silicon reactor and a novel mid-IR fiber-optic flow cell were used for the enzymatic determination of sucrose in aqueous solution. The method relies on the enzymatic hydrolysis of sucrose to fructose and glucose catalyzed by β-fructosidase and on the acquisition of FT-IR spectra before and after complete reaction. β-Fructosidase was covalently bound to the porous silicon surface of the channels in the microreactor. The porous silicon was achieved by anodization of the silicon reactor in a HF/ethanol mixture. For the measurement of small amounts of aqueous solution, a miniaturized flow cell was developed which consisted of two AgCl(x)Br(1)(-)(x) fiber tips (diameter, 0.75 mm) coaxially mounted in a PTFE block at a distance of 23 μm. The flowing stream was directed through the gap of the two fiber tips which served to define the optical path length and to bring the focused mid-IR radiation to the place of measurement. Using this construction, a probed volume of ～10 nL was obtained. The calibration curve was linear between 10 and 100 mmol/L sucrose. Furthermore, the potential of this method was demonstrated by the analysis of binary sucrose/glucose mixtures showing no interference from glucose and by the successful determination of sucrose in real samples.     

Freestanding silicon quantum dots: origin of red and blue luminescence

In this paper, we studied the behavior of silicon quantum dots (Si-QDs) after etching and surface oxidation by means of photoluminescence (PL) measurements, Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance spectroscopy (EPR). We observed that etching of red luminescing Si-QDs with HF acid drastically reduces the concentration of defects and significantly enhances their PL intensity together with a small shift in the emission spectrum. Additionally, we observed the emergence of blue luminescence from Si-QDs during the re-oxidation of freshly etched particles. Our results indicate that the red emission is related to the quantum confinement effect, while the blue emission from Si-QDs is related to defect states at the newly formed silicon oxide surface.     

Graphene masks as passivation layers in the electrochemical etching of silicon

Site-specific masking with graphene films has the potential to facilitate low-cost, high-throughput micro-patterns on silicon substrates over large areas. Here, a facile approach to using graphene as a masking agent on silicon wafers for site-specific patterning is demonstrated. Graphene sheets were deposited via a sealing-tape-exfoliation method onto hydride-terminated (Si–H) silicon substrates. Raman confocal mapping showed inhibition of oxidation of the Si wafer underlying the graphene, indicating that the graphene restricts the diffusion of oxygen onto the Si surface. The graphene coated Si substrates were then electrochemically etched in an aqueous HF/ethanol (3:1 (v/v)) anodization solution. Scanning electron microscopy showed that the graphene layer successfully restricted the etching of the Si surface, however, near the edge sites of the graphene deep etching occurred.     

Self-ordering of anodic nanoporous alumina fabricated by accelerated mild anodization method

A processing method called “accelerated mild anodization” is developed for preparation of high density and uniform nanoporous by anodization of aluminum. The idea is to use two different temperatures for both sides of sample in order to maintain mid level of current density during the anodization process. Here we have used high temperature for the back side of the sample in order to increase the current density while the electrolyte is kept at low temperature in the level of mild anodization. It is shown that not only the film growth is considerably fast, almost ten times faster than mild anodization, but also the anodization voltage is constant and anodization current variation is much less compared to hard anodization technique. Using oxalic acid, interpore distances of 89, 104, 117 and 130 nm were obtained for 35, 40, 45 and 50 V anodization voltages, respectively. It is found that the interpore distances are proportional to the anodization potential, almost same as that for the mild anodization. The porosity obtained tended to obey the same rule as that in mild anodization. This method is promising for industrial application due to short fabrication time as well as high-speed pore ordering.     

直流电化学腐蚀法制备多孔硅的表面形貌研究

采用正交实验，直流电化学腐蚀法制备多孔硅。用原子力显微镜对表面进行观察，研究电化学腐蚀参数对其表面形貌的影响。氢氟酸浓度（CHF）升高，使临界电流密度（JPS）增大，有利于多孔硅的形成。电流密度（J）增大，多孔硅的孔隙率和孔径随之变大，而其纳米粒径将变小。腐蚀时间（t）越长，孔径越大，孔越深。