Formation of aligned silicon nanowire on silicon by electroless etching in HF solution

It was demonstrated that the etching in HF-based aqueous solution containing AgNO₃ and Na₂S₂O₈ as oxidizing agents or by Au-assisted electroless etching in HF/H₂O₂ solution at 50 °C yields films composed of aligned Si nanowire (SiNW). SiNW of diameters ∼10 nm were formed. The morphology and the photoluminescence (PL) of the etched layer as a function of etching solution composition were studied. The SiNW layers formed on silicon were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and photoluminescence. It was demonstrated that the morphology and the photoluminescence of the etched layers strongly depends on the type of etching solution. Finally, a discussion on the formation process of the silicon nanowires is presented.     

Structural and optical properties of thin films porous amorphous silicon carbide formed by Ag-assisted photochemical etching

In this work, we present the formation of porous layers on hydrogenated amorphous SiC (a-SiC: H) by Ag-assisted photochemical etching using HF/K₂S₂O₈ solution under UV illumination at 254 nm wavelength. The amorphous films a-SiC: H were elaborated by d.c. magnetron sputtering using a hot pressed polycrystalline 6H-SiC target. Because of the high resistivity of the SiC layer, around 1.6 MΩ cm and in order to facilitate the chemical etching, a thin metallic film of high purity silver (Ag) has been deposited under vacuum onto the thin a-SiC: H layer. The etched surface was characterized by scanning electron microscopy, secondary ion mass spectroscopy, infrared spectroscopy and photoluminescence. The results show that the morphology of etched a-SiC: H surface evolves with etching time. For an etching time of 20 min the surface presents a hemispherical crater, indicating that the porous SiC layer is perforated. Photoluminescence characterization of etched a-SiC: H samples for 20 min shows a high and an intense blue PL, whereas it has been shown that the PL decreases for higher etching time. Finally, a dissolution mechanism of the silicon carbide in 1HF/1K₂S₂O₈ solution has been proposed.     

Au-assisted electroless etching of silicon in aqueous HF/H2O2 solution

The Au-assisted electroless etching of p-type silicon substrate in HF/H₂O₂ solution at 50 °C was investigated. The dependence of the crystallographic orientation, the concentration of etching solution and the silicon resistivity on morphology of etched layer was studied. The layers formed on silicon were investigated by scanning electron microscopy (SEM). It was demonstrated that although the deposited Au on silicon is a continuous film, it can produce a layer of silicon nanowires or macropores depending on the used solution concentration.     

Chemical etching investigation of polycrystalline p-type 6H-SiC in HF/Na2O2 solutions

In this work, an experimental study on the chemical etching reaction of polycrystalline p-type 6H-SiC was carried out in HF/Na₂O₂ solutions. The morphology of the etched surface was examined with varying Na₂O₂ concentration, etching time, agitation speed and temperature. The surfaces of the etched samples were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) Fourier transform infrared spectroscopy (FT-IR) and photoluminescence. The surface morphology of samples etched in HF/Na₂O₂ is shown to depend on the solution composition and bath temperature. The investigation of the HF/Na₂O₂ solutions on 6H-SiC surface shows that as Na₂O₂ concentration increases, the etch rate increases to reach a maximum value at about 0.5 M and then decreases. A similar behaviour has been observed when temperature of the solution is increased. The maximum etch rate is found for 80 °C. In addition, a new polishing etching solution of 6H-SiC has been developed. This result is very interesting since to date no chemical polishing solution has been developed on the material.     

Effect of temperature and silicon resistivity on the elaboration of silicon nanowires by electroless etching

The morphology of silicon nanowire (SiNW) layers formed by Ag-assisted electroless etching in HF/H₂O₂ solution was studied. Prior to the etching, the Ag nanoparticles were deposited on p-type Si(1 0 0) wafers by electroless metal deposition (EMD) in HF/AgNO₃ solution at room temperature. The effect of etching temperature and silicon resistivity on the formation process of nanowires was studied. The secondary ion mass spectra (SIMS) technique is used to study the penetration of silver in the etched layers. The morphology of etched layers was investigated by scanning electron microscope (SEM).     

Oxidizing agent concentration effect on metal-assisted electroless etching mechanism in HF-oxidizing agent-H2O solutions

The mechanism of metal-assisted electroless etching of silicon in HF-oxidizing agent-H₂O etching system as a function of oxidizing agent concentration was studied. Three types of oxidizing agent were experimented namely Na₂S₂O₈, K₂Cr₂O₇ and KMnO₄. Their concentrations were varied from 0.05 M to 0.3 M. The layers formed on silicon were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray (EDX). It is shown that an insoluble solid-phase film (K₂SiF₆) form on silicon surface when concentration of K₂Cr₂O₇ or KMnO₄ increases in chemical solutions. On other hand, when Na₂S₂O₈ concentration increases, the surface roughness decreases without any chemical complex formation.     

Preparation of Bi2S3 thin films with a nanoleaf structure by electrodeposition method

Nanoleaf-like Bi₂S₃ thin films were deposited on indium tin oxide (ITO) glass using Bi(NO₃)₃ and Na₂S₂O₃ as precursors by a cathodic electrodeposition process. The as-deposited thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and photoluminescence spectrum (PL). The influence of precursor solution mole concentration ratios [Bi(NO₃)₃]/[Na₂S₂O₃] on the phase compositions, morphologies and photoluminescence properties of the obtained thin films were investigated. Results show that a uniform Bi₂S₃ thin film with nanoleaf structure can be obtained with the precursor solution concentration ratio [Bi(NO₃)₃]/[Na₂S₂O₃] = 1:7. The as-prepared thin films exhibit blue-green photoluminescence properties under ultraviolet light excitation. With the increase of concentration ratios [Bi(NO₃)₃]/[Na₂S₂O₃] in the deposition solution, the crystallizations and PL properties of Bi₂S₃ thin films are obviously improved.     

Influence of [S2O3]/[Sm] on Sm2S3 thin films deposited by liquid phase deposition method on self-assembled monolayers

Sm₂S₃ thin films were prepared on Si (1 0 0) substrates using SmCl₃ and Na₂S₂O₃ as precursors by liquid phase deposition method on self-assembled monolayers. The influence of the molar concentration ratio of [S₂O₃²⁻]/[Sm³⁺] on the phase compositions, surface morphologies and optical properties of the as-deposited films were investigated. The as-deposited Sm₂S₃ thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), ultraviolet-visible (UV-vis) and photoluminescence spectrum (PL). Results show that it is important to control the [S₂O₃²⁻]/[Sm³⁺] during the deposition process and monophase Sm₂S₃ thin films with orientation growth along (0 1 1) direction can be achieved when [S₂O₃²⁻]/[Sm³⁺] = 2.0, pH 3.0, with citric acid as a template agent. The as-deposited thin films exhibit a dense and crystalline surface morphology. Good transmittance in the visible spectrum and excellent absorbency of ultraviolet light of the thin films are observed, and the band gap of the thin films first decrease and then increase with the increase of the [S₂O₃²⁻]/[Sm³⁺]. The as-deposited thin films also exhibit red photoluminescence properties under visible light excitation. With the increase of the [S₂O₃²⁻]/[Sm³⁺] in the deposition solution, the PL properties of Sm₂S₃ thin films are obviously improved.     

Characteristic properties of Y2SiO5:Ce thin films grown with PLD

Three different gases (nitrogen (N₂), oxygen (O₂) and argon (Ar)) were used as background gases during the growth of pulsed laser deposition (PLD) Y₂SiO₅:Ce thin films. A Krypton fluoride laser (KrF), 248 nm was used for the PLD of the films on silicon (Si) (1 0 0) substrates. The effect of the background gases on the surface morphology, crystal growth and luminescent properties were investigated. All the experimental parameters, the gas pressure (455 mT), the substrate temperature (600 °C), the pulse frequency (8 Hz), the number of pulses (4000) and the laser fluence (1.6±0.2) J/cm² were kept constant. The only parameter that was changed during the deposition was the ambient gas species. The surface morphology and average particle sizes were monitored with scanning electron microscopy (SEM) and atomic force microscopy (AFM). X-ray diffraction (XRD) and Auger electron spectroscopy (AES) were used to determine the crystal structure and composition, respectively. Cathodo- (CL) and photoluminescence (PL) were used to measure the luminescent intensities for the different phosphor thin films. The nature of the particles, ablated on the substrate, is related to the collisions between the ejected particles and the ambient gas particles. The CL and PL intensities also depend on the particle sizes. A 144 h (coulomb dose of 1.4×10⁴ C cm⁻²) electron degradation study on the thin films ablated in the Ar gas environment resulted in a decrease in the main CL intensity peak at 440 nm and to the development of a new very broad luminescent peak spectra ranging from 400 to 850 nm due to the growth of a SiO₂ layer on the surface.     

Fabrication, morphology, and photoluminescence properties of GaN nanowires and nanorods by ammoniating Ga2O3/V films on Si(1 1 1)

GaN nanowires and nanorods have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/V films at 900 °C in a quartz tube. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectrum were carried out to characterize the structure, morphology, and photoluminescence properties of GaN sample. The results show that the GaN nanowires and nanorods with pure hexagonal wurtzite structure have good emission properties. The growth direction of nanostructures is perpendicular to the fringes of (1 0 1) plane. The growth mechanism is also briefly discussed.     

Morphology and optical properties of p-type porous GaAs(1 0 0) layers made by electrochemical etching

Porous GaAs layers were formed by electrochemical etching of p-type GaAs(1 0 0) substrates in HF solution. A surface characterization has been performed on p-type GaAs samples using X-ray photoelectron spectroscopy (XPS) technique in order to get information about the chemical composition, particularly on the surface contamination. According to the XPS spectra, the oxide layer on as-received porous GaAs substrates contains As₂O₃, As₂O₅ and Ga₂O₃. Large amount of oxygen is present at the surface before the surface cleaning.Compared to untreated GaAs surface, room temperature photoluminescence (PL) investigations of the porous layers reveal the presence of two PL bands: a PL peak at ∼871 nm and a “visible” PL peak at ∼650-680 nm. Both peak wavelengths and intensities varied from sample to sample depending on the treatment that the samples have undergone. The short PL wavelength at 650-680 nm of the porous layers is attributed to quantum confinement effects in GaAs nano-crystallites. The surface morphology of porous GaAs has been studied using atomic force microscopy (AFM). Nano-sized crystallites were observed on the porous GaAs surface. An estimation of the mean size of the GaAs nano-crystals obtained from effective mass theory and based on PL data was close to the lowest value obtained from the AFM results.     

Crystallographic and luminescent characterizations of blue-emitting BaAl2S4 : Eu electroluminescent thin films

We characterized the crystallization and luminescence of blue-emitting BaAl₂S₄ : Eu electroluminescent thin films, prepared using switching electron-beam evaporation with two targets. From the photoluminescence intensity and decay profile of the activated Eu²⁺ ions in the BaAl₂S₄ : Eu, we found that the optimum annealing conditions for preparing highly luminescent thin films are a temperature of around 900°C and an annealing time of 2 min. We analyzed the crystalline properties using cross-sectional transmission electron microscope images. Evaluation of the cathodoluminescence spectra in the cross-sections showed that the BaAl₂S₄ : Eu emitting layer was luminously inhomogeneous on the depth of the layer and that the main luminescent area was near the surface of the emitting layer. We discuss here the relationship between the crystalline and luminescent properties.     

Enhanced emission of Er from alternately Er doped Si-rich Al2O3 multilayer film with Si nanocrystals as broadband sensitizers

Alternately Er doped Si-rich Al₂O₃ (Er:SRA) multilayer film, consisting of alternate Er-Si-codoped Al₂O₃ (Er:Si:Al₂O₃) and Si-doped Al₂O₃ (Si:Al₂O₃) sublayers, has been synthesized by co-sputtering from separated Er, Si, and Al₂O₃ targets. The dependence of Er³⁺ related photoluminescence (PL) properties on annealing temperatures over 700-1100 °C was studied. The maximum intensity of Er³⁺ PL, about 10 times higher than that of the monolayer film, was obtained from the multilayer film annealed at 950 °C. The enhancement of Er³⁺ PL intensity is attributed to the energy transfer from the silicon nanocrystals in the Si:Al₂O₃ sublayers to the neighboring Er³⁺ ions in the Er:Si:Al₂O₃ sublayers. The PL intensity exhibits a nonmonotonic temperature dependence: with increasing temperature, the integrated intensity almost remains constant from 14 to 50 K, then reaches maximum at 225 K, and slightly increases again at higher temperatures. Meanwhile, the PL integrated intensity at room temperature is about 30% higher than that at 14 K.     

Enhancement of photoluminescence intensity from Si nanodots using Al2O3 surface passivation layer grown by atomic layer deposition

Temperature-dependent photoluminescence (PL) from Si nanodots with Al₂O₃ surface passivation layers was studied. The Si nanodots were grown by low pressure chemical vapor deposition and the Al₂O₃ thin films were prepared by atomic layer deposition (ALD), respectively. The BOE (Buffer-Oxide-Etch) treatment resulted in the damaged surface of Si nanodots and thus caused dramatic reduction in the PL intensity. Significant enhancement of the PL intensity from Si nanodots after the deposition of Al₂O₃ thin films was observed over a wide temperature range, indicating the remarkable surface passivation effect to suppress the non-radiative recombination at the surface of Si nanodots. The results demonstrated that the Al₂O₃ surface passivation layers grown by ALD are effectually applicable to nanostructured silicon devices.     

Dependences of the surface and the optical properties on the O2/O2 + Ar flow-rate ratios for ZnO thin films grown on ZnO buffer layers

ZnO active layers on ZnO buffer layers were grown at various O₂/O₂ + Ar flow-rate ratios by using radio-frequency magnetron sputtering. Atomic force microscopy images showed that the surface roughnesses of the ZnO active layers grown on ZnO buffer layers decreased with decreasing O₂ atmosphere, indicative of an improvement in the ZnO surfaces. The type of the ZnO active layer was n-type, and the resistivity of the layer increased with increasing O₂ atmosphere. Photoluminescence spectra from the ZnO active layers grown on the ZnO buffer layers showed dominant peaks corresponding to local levels in the ZnO energy gap resulting from oxygen vacancies or interstitial zinc vacancies, and the peak positions changed significantly with the O₂/O₂ + Ar flow rate. These results can help improve understanding of the dependences of the surface and the optical properties on the O₂/O₂ + Ar ratio for ZnO thin films grown on ZnO buffer layers.     

Visible luminescence of Al2O3 nanoparticles embedded in silica glass host matrix

This paper deals with the sol-gel elaboration and defects photoluminescence (PL) examination of Al₂O₃ nanocrystallites (size ∼30 nm) confined in glass based on silica aerogel. Aluminium oxide aerogels were synthesized using esterification reaction for hydrolysis of the precursor and supercritical conditions of ethyl alcohol for drying. The obtained nanopowder was incorporated in SiO₂ host matrix. After heating under natural atmosphere at 1150 °C for 2 h, the composite Al₂O₃/SiO₂ (AS) exhibited a strong PL bands at 400-600 and 700-900 nm in 78-300 K temperature range. PL excitation (PLE) measurements show different origins of the emission. It was suggested that OH-related radiative centres and non-bridging oxygen hole centres (NBOHCs) were responsible for the bands at 400-600 and 700-900 nm, respectively.     

Nanocrystal and interface defects related photoluminescence in silicon-rich Al2O3 films

In this study, silicon nanocrystal-rich Al₂O₃ film has been prepared by co-sputtering a silicon and alumina composite target and subsequent annealing in N₂ atmosphere. The microstructure of the film has been characterized by infrared (IR) absorption, Raman spectra and UV-absorption spectra. Typical nanocrystal and interface defects related photoluminescence with the photon energy of 1.54 (IR band) and 1.69 eV (R band) has been observed by PL spectrum analysis. A post-annealing process in oxygen atmosphere has been carried out to clarify the emission mechanism. Despite the red shift of the spectra, enhanced emission of the 1.69 eV band together with the weak emission phenomenon of the 1.54 eV band has been found after the post-annealing. The R band is discussed to originate from silicon nanocrystal interface defects. The IR band is concluded to be a coupling effect between electronic and vibrational emissions.     

Synthesis and structure of the monolayer hydrate K0.3CoO2·0.4H2O

The monolayer hydrate (MLH) K_0.3CoO₂·0.4H₂O was synthesized from K_0.6CoO₂ by extracting K⁺ cations using K₂S₂O₈ as an oxidant and the subsequent intercalation of water between the layers of edge-sharing CoO₆ octahedra. A hexagonal structure (space group P6₃/mmc) with lattice parameters a=2.8262(1) Å, c=13.8269(6) Å similar to the MLH Na_0.36CoO₂·0.7H₂O was established using high-resolution synchrotron X-ray powder diffraction data. The K/H₂O layer in the K-MLH is disordered, which is in contrast to the Na-MLH. At low temperatures metallic and paramagnetic behavior was found.     

The effect of fluorine-based plasma treatment on morphology and chemical surface composition of biocompatible silicone elastomer

X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) have been used to investigate the effect of reactive ion etching (RIE) on poly(methylhydrogensiloxane-co-dimethylsiloxane) surface in fluorine-based plasmas. Polysiloxane layers supported on the standard silicon wafers were etched using SF₆ + O₂ or CF₄ + O₂ plasmas. SEM studies show that the polysiloxane morphology depends on plasma chemical composition strongly. Presence of a columnar layer likely covered with a fluorine rich compound was found on the elastomer surface after the CF₄ + O₂ plasma exposure. After the SF₆ + O₂ or CF₄ + O₂ plasma treatment the polysiloxane surface enriches with fluorine or with fluorine and aluminum, respectively. Different morphologies and surface chemical compositions of the silicone elastomer etched in both plasmas indicate different etching mechanisms.     

Nanocrystalline ZnO film deposited by ultrasonic spray on textured silicon substrate as an anti-reflection coating layer

A ZnO thin film was successfully synthesized on glass, flat surface and textured silicon substrates by chemical spray deposition. The textured silicon substrate was carried out using two solutions (NaOH/IPA and Na₂CO₃). Textured with Na₂CO₃ solution, the sample surface exhibits uniform pyramids with an average height of 5 μm. The properties and morphology of ZnO films were investigated. X-ray diffraction (XRD) spectra revealed a preferred orientation of the ZnO nanocrystalline film along the c-axis where the low value of the tensile strain 0.26% was obtained. SEM images show that all films display a granular, polycrystalline morphology. The morphology of the ZnO layers depends dramatically on the substrate used and follows the contours of the pyramids on the substrate surface. The average reflectance of the textured surface was found to be around 13% and it decreases dramatically to 2.57% after deposition of a ZnO antireflection coating. FT-IR peaks arising from the bonding between Zn–O are clearly represented using a silicon textured surface. A very intense photoluminescence (PL) emission peak is observed for ZnO/textured Si, revealing the good quality of the layer. The PL peak at 380.5 nm (UV emission) and the high-intensity PL peak at 427.5 nm are observed and a high luminescence occurs when using a textured Si substrate.