Electrochemical Etching Of Si Research Articles

Fabrication of optically smooth surface on the cleavage of porous silicon by gas cluster ion irradiation

Electrochemical etching of Si is a promising method of fabricating multilayer photonic structures with hundreds of layers. Nevertheless, the natural cleavage of the porous silicon structure has roughness and then high optical scattering that embarrasses the usage of porous silicon in optical devices. In this study, gas cluster ion beam irradiation was suggested as a polishing technique. The bombardment results in surface smoothing and consequent enhancement of light reflection without significant difference in sputtering rate of layers with different porosity. Increasing irradiation dose results in different behavior of porous layers: densification of low-porosity structures and nano-rods formation in high-porosity layers.

Metal assisted anodic etching of silicon.

Metal assisted anodic etching (MAAE) of Si in HF, without H2O2, is demonstrated. Si wafers were coated with Au films, and the Au films were patterned with an array of holes. A Pt mesh was used as the cathode while the anodic contact was made through either the patterned Au film or the back side of the Si wafer. Experiments were carried out on P-type, N-type, P(+)-type and N(+)-type Si wafers and a wide range of nanostructure morphologies were observed, including solid Si nanowires, porous Si nanowires, a porous Si layer without Si nanowires, and porous Si nanowires on a thick porous Si layer. Formation of wires was the result of selective etching at the Au-Si interface. It was found that when the anodic contact was made through P-type or P(+)-type Si, regular anodic etching due to electronic hole injection leads to formation of porous silicon simultaneously with metal assisted anodic etching. When the anodic contact was made through N-type or N(+)-type Si, generation of electronic holes through processes such as impact ionization and tunnelling-assisted surface generation were required for etching. In addition, it was found that metal assisted anodic etching of Si with the anodic contact made through the patterned Au film essentially reproduces the phenomenology of metal assisted chemical etching (MACE), in which holes are generated through metal assisted reduction of H2O2 rather than current flow. These results clarify the linked roles of electrical and chemical processes that occur during electrochemical etching of Si.

Structural, Surface Morphological and Photoluminescence Properties of Nanostructured Porous Silicon Material for Optoelectronics Application

The promotion of silicon (Si) from being the key material for microelectronics to an interesting material for optoelectronic application is a consequence of the possibility to reduce its device dimensionally by a cheap and easy technique. In fact, electrochemical etching of Si under controlled conditions leads to the formation of nanocrystalline porous silicon (PS) where quantum confinement of photo excited carriers and surface species yield to a band gap opening and an increased radiative transition rate resulting in efficient light emission. In the present study, the nanostructured PS samples were prepared using anodic etching of p-type silicon. The effect of current density on structural and optical properties of PS, has been investigated. XRD studies confirm the presence of silicon nanocrystallites in the PS structure. By increasing the current density, the average estimated values of grain size are found to be decreased. SEM images indicate that the pores are surrounded by a thick columnar network of silicon walls. The observed PL spectra at room temperature for all the current densities confirm the formation of PS structures with nanocrystalline features. PL studies reveal that there is a prominent visible emission peak at 606 nm. The obtained variation of intensity in PL emission may be used for intensity varied light emitting diode applications. These studies confirm that the PS is a versatile material with potential for optoelectronics application.

Surface Engineered Porous Silicon-based Nanostructures for Cancer Therapy

ABSTRACTIn this work, nanostructured porous silicon (PSi) hosts, synthesized by electrochemical etching of Si, are designed to carry and release the anti cancer drug, mitoxantrone dihydrochloride (MTX). We study the effect of surface chemistry of the Si scaffold on its properties as a drug carrier. The freshly-etched PSi is modified by surface alkylation using thermal hydrosilylation with 1-dodecene. Fourier-transform infrared spectroscopy and nitrogen adsorption-desorption measurements are employed to characterize the PSi carriers after chemical modification. Both, drug loading efficiency and release kinetics are found to be significantly affected by surface chemistry of the PSi. In vitro cytotoxicity studies on human breast carcinoma (MDA-MB-231) cells show that the MTX released from the PSi hosts maintains its cytotoxic functionality.

Spectroscopic investigations of the stability of porous silicon structure obtained by etching Si(100) in aqueous ammonium fluoride solution

The structure of porous silicon (por-Si) obtained by electrochemical etching of Si(100) single crystal wafers in an aqueous ammonium fluoride solution with isopropyl alcohol additions has been studied using X-ray reflection spectroscopy, X-ray photoelectron spectroscopy, and external X-ray quantum yield measurements. It is established that por-Si layers obtained by the nontraditional technology (not involving hydrofluoric acid) possess a partly amorphized structure and bear a stable surface oxide film with a thickness not exceeding 5 nm.

Vertical hybrid microcavity based on a polymer layer sandwiched between porous silicon photonic crystals

A vertical hybrid microcavity is fabricated by sandwiching a polymer layer between distributed Bragg reflectors (DBRs) composed of porous silicon photonic crystals. The DBRs are made by electrochemical etching of Si and consist of alternating porous Si layers of high and low porosity, the top DBR being a freestanding film. The hybrid microcavity demonstrates a deep microcavity mode placed within a 200 nm wide photonic band gap, and reveals a many-fold enhancement of the third-order nonlinearity of the microcavity layer. The fabrication technique employed is rather simple, enabling the use of a variety of functional materials as the microcavity spacer.

Highly ordered hexagonally arranged nanostructures on silicon through a self-assembledsilicon-integrated porous anodic alumina masking layer

A combined process of electrochemical formation of self-assembled porous anodic aluminathin films on a Si substrate and Si etching through the pores was used to fabricate ideallyordered nanostructures on the silicon surface with a long-range, two-dimensionalarrangement in a hexagonal close-packed lattice. Pore arrangement in the alumina film wasachieved without any pre-patterning of the film surface before anodization. Perfect patterntransfer was achieved by an initial dry etching step, followed by wet or electrochemicaletching of Si at the pore bottoms. Anisotropic wet etching using tetramethyl ammoniumhydroxide (TMAH) solution resulted in pits in the form of inverted pyramids, whileelectrochemical etching using a hydrofluoric acid (HF) solution resulted in concave nanopitsin the form of semi-spheres. Nanopatterns with lateral size in the range 12–200 nm, depth inthe range 50–300 nm and periodicity in the range 30–200 nm were achieved either onlarge Si areas or on pre-selected confined areas on the Si substrate. The poresize and periodicity were tuned by changing the electrolyte for porous anodicalumina formation and the alumina pore widening time. This parallel large-areananopatterning technique shows significant potential for use in Si technology anddevices.

RuO 2/SiO 2/Si and SiO 2/porous Si/Si interfaces analysed by SIMS

Compared are interfaces between thin silicon oxide layers and two substrates bulk silicon and porous silicon. About 2 μm thick porous silicon was prepared by an electrochemical etching of Si in HF solution. Oxide layers were formed by thermal oxidation. Part of SiO 2/Si samples were covered with 50 nm RuO 2 oxide. SIMS depth profiles of these interfaces were performed using ultra-low energy (880 eV) and low energy (5 keV) argon ion beams at several incidence angles. Positive and negative secondary ions were analysed by quadrupole mass spectrometer QMA 410 Balzers during Ar + bombardment with the use of 06-350E Physical Electronics ion gun. Charge build-up effects during positive secondary ion detection were observed dependent of the thickness of silicon oxide layer. The SiO 2/Si interface regions characterised by SIMS show differences related to the kind of a substrate used. In case of 50 nm SiO 2/porous Si interface, characteristic oxygen depletion region (20 nm thick) is present on the porous silicon side. The observed depletion was confirmed also by Auger depth profiling. The depletion formation mechanism is explained on the basis of ion beam induced redeposition of oxygen inside the pores.

The mechanism of Si etching in fluoride solutions

Several mechanisms for (photo)electrochemical etching of Si in aqueous fluoride solutions have been proposed. These models are reviewed and quantitative aspects of the kinetics and energetics of these mechanisms are evaluated. In particular, the roles of HF, F−, HF2−, OH− and H2O in etching both in the dark and under illumination are considered. The presence of a hole in a bulk band (as opposed to a surface state/resonance associated with the Si–H bond or the Si–Si backbond) initiates (photo)electrochemical etching. The sticking coefficient of F−(aq) ions on H-terminated Si increases by 11 orders of magnitude in the presence of this hole. The sticking coefficients of HF(aq) and HF2− (aq) in the course of the rate determining step are also calculated. The possible involvement of abstraction reactions within the overall mechanism is discussed. A modified reaction mechanism, which is consistent with the kinetic and energetic parameters discussed here, is presented.

Study on hydrogen reactivity with surface chemical species of nanocrystalline porous silicon

The nanocrystalline porous silicon was prepared by the electrochemical etching of Si in a HF solution. A semi-transparent palladium layer was thermally deposited on its surface. Micro-Raman spectra were recorded in air and in the presence of hydrogen. Different SiH X stretching and wagging vibrational modes, a SiH stretching mode modified by the presence of three oxygen atoms in the Si backbonds (O 3SiH unit) and a SiOSi vibration were readily revealed. Their behavior under laser irradiation and towards the hydrogen atoms dissociated from hydrogen molecules by palladium is reported.

Photoluminescence and chemical bonding in porous silicon layers — dependence on the P concentration in the Si substrate

Correlation between the photoluminescence (PL), porosity, crystallite size and chemical bonds in porous silicon (PS) prepared by electrochemical etching of Si with different levels of phosphorus doping has been studied. PL is very strongly related to the chemical bonding and depends on the Si, O and C content which is different for PS prepared on Si substrates with different resistivities. The influence of the doping level of Si on the process of electrochemical etching is discussed.

Electrochemical etching of Si(001) in NH4F solutions: Initial stage and {111} microfacet formation

In situ scanning tunneling microscopy (STM) has been used to examine the etching of an n-Si(001) electrode in 0.1 M NH4F. Cathodic polarization facilitated chemical etching of Si(001) to give {111} microfacets as a result of the tendency of Si to form a monohydride terminated surface. Time-dependent in situ STM atomic images were obtained to demonstrate the preferential etching at the kinks and steps. From the results of the time-dependent imaging, local etching rates were evaluated for the specific crystallographic directions. A Si(001):H-(1×1) square structure was also obtained, demonstrating the presence of dihydride configuration in the beginning of the etching.