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Abstract
Porous silicon (PS) offers many potential advantages for the realization of optoelectronic applications. However, the electrochemical anodizing method used to fabricate porous silicon has resulted in many undesirable sub-effects, such as impurities due to reactive residents, nonstabilized surface, nonuniform pore structure, and high film stress, leading to contamination of conventional VLSI processes. In this paper, we report a new dry etching method to produce the light-emitting PS. The formation mechanism of this film is proposed. Using the samples prepared with this new method, results demonstrate that the luminescence in this film does not have strong correlation with size of the nano-structures and we therefore suggest that the luminescence in this film is not due to the quantum confinement effects
... It is fabricated when crystalline silicon wafers are etched photoelectrochemically in hydrofluoric acid HF-based electrolyte sol this method can be controlled through several parameters such as current density, power density, etching time and Hf concentration, etc. [1]. It is a promising material due to the excellent optical, mechanical, thermal properties, chemical stability and the low cost [2]; therefore has a wide range of potential application like photovoltaic devises, chemical sensors, biological sensors 1D photonic crystals, etc. [3][4][5][6][7]. Photoelectrochemical etching an easy method, where light or laser illuminated the silicon electrode during the anodization procedure. ...
... It is fabricated when crystalline silicon wafers are etched photoelectrochemically in hydrofluoric acid HF based electrolyte sol this method can be controlled through several parameters such as current density, power density, etching time and Hf concentration etc. [4]. It is a promising material due to the excellent optical, mechanical, thermal properties, chemical stability and the low cost [5] therefore, has a wide range of potential application like photovoltaic devises, chemical sensors, biological sensors etc. [6][7][8][9][10]. Laser assisted etching (LAE) an easy method and no thermal effect, where light or laser illuminated the silicon electrode during the anodization procedure. ...
... It is fabricated when crystalline silicon wafers are etched photoelectrochemically in hydrofluoric acid HF based electrolyte sol this method can be controlled through several parameters such as current density, power density, etching time and Hf concentration etc. [4]. It is a promising material due to the excellent optical, mechanical, thermal properties, chemical stability and the low cost [5] therefore, has a wide range of potential application like photovoltaic devises, chemical sensors, biological sensors etc. [6][7][8][9][10]. Laser assisted etching (LAE) an easy method and no thermal effect, where light or laser illuminated the silicon electrode during the anodization procedure. ...
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Laser-assisted etching process was used to prepare porous silicon on n-type silicon substrate of resistivity 10 Ω cm and <100> orientation incubated in hydrofluoric acid of 25% concentration. The porous layers were prepared using laser diode of 645 nm wavelength illumination with different intensities of about 14 and 20 mW/cm2 and current density of 10 mA/cm2 for a fixed etching time (8 min). The morphology and optical characterization of porous silicon were evaluated. Scanning electron microscopy results demonstrated that the power density is a prominent parameter that controls the morphology of pores. The reflectivity decreased with increase in the highest power density to 0.66% and the peak position of PL results increased with increase in power density and moved toward the blue region. These phenomena were due to changing in the characteristics of the surface morphology the change of Si to nanostructures.
A novel plasma hydrogenation method for the fabrication of nano-crystalline structures of silicon as well as the photoluminescence and structural properties of these porous structures is presented. We have observed that the hydrogenation process followed by an annealing treatment results in the formation of nano-crystalline silicon structures where increased temperatures during hydrogenation reduces the grain size. Furthermore, by increasing the time of the hydrogenation process, the density of the silicon grains is increased.
Photoluminescence (PL) spectroscopy demonstrated the presence of a direct gap in the visible light range where materials with a smaller grain size emitted light at lower wavelengths, and a higher density of grains resulted in higher amplitudes in the PL spectrum. TEM and SEM characterization of these samples and the structure-emission relationship are also presented.
The preparation of porous silicon films by DC-plasma hydrogenation and subsequent annealing of amorphous silicon films on silicon and glass substrates is reported for the first time. The effects of varying plasma power and annealing temperatures have been investigated and characterized by scanning-electron microscopy, transmission-electron microscopy, and photoluminescence. A plasma density of about 5.5 W/m2 and hydrogenation-annealing temperatures of about 400 °C was found to be suitable for the formation of nano-crystalline silicon films with grain diameters of the order of 3–10 nm. The intensity and wavelength of the emitted visible light were found to depend on the hydrogenation and annealing conditions, and patterning of the silicon films using standard lithography allowed the creation of light-emitting patterns.
LIGHT-emitting devices based on silicon would find many applications in both VLSI and display technologies, but silicon normally emits only extremely weak infrared photoluminescence because of its relatively small and indirect band gap1. The recent demonstration of very efficient and multicolour (red, orange, yellow and green) visible light emission from highly porous, electrochemically etched silicon2,3 has therefore generated much interest. On the basis of strong but indirect evidence, this phenomenon was initially attributed to quantum size effects within crystalline material2, but this interpretation has subsequently been extensively debated. Here we report results from a transmission electron microscopy study which reveals the structure of the porous layers that emit red light under photoexcitation. Our results constitute direct evidence that highly porous silicon contains quantum-size crystalline structures responsible for the visible emission. We show that arrays of linear quantum wires are present and obtain images of individual quantum wires of width <3 nm.
Indirect evidence is presented that free‐standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography. The novel approach uses electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers. Mesoporous Si layers of high porosity exhibit visible (red) photoluminescence at room temperature, observable with the naked eye under ≪1 mW unfocused (≪0.1 W cm<sup>-2</sup>) green or blue laser line excitation. This is attributed to dramatic two‐dimensional quantum size effects which can produce emission far above the band gap of bulk crystalline Si.
The introduction of solvents into the pores of optical waveguides formed using porous silicon is shown to dramatically reduce the interfacial scattering loss of the waveguides (by as much as 34-dB cm/sup -1/ in one example), in a reversible manner. The degree of loss reduction is dependent on the type of solvent introduced. These observations, combined with the fact that a substantial portion of the guided-mode field interacts with the solvent introduced into the pores, indicate that an enhanced sensitivity for sensor applications may be achievable across a broad range of operational wavelengths.
A biosensor has been developed based on induced wavelength shifts in the Fabry-Perot fringes in the visible-light reflection spectrum of appropriately derivatized thin films of porous silicon semiconductors. Binding of molecules induced changes in the refractive index of the porous silicon. The validity and sensitivity of the system are demonstrated for small organic molecules (biotin and digoxigenin), 16-nucleotide DNA oligomers, and proteins (streptavidin and antibodies) at pico- and femtomolar analyte concentrations. The sensor is also highly effective for detecting single and multilayered molecular assemblies.
— Recently discovered visible-emitting porous silicon can potentially lead to many advanced optoelectronic devices, including a new class of flat-panel displays which can feature monolithic integration of light-emitting pixels and associated driver electronics on a single substrate, using relatively simple and cost-effective fabrication processes. Our research group has prepared porous-silicon layers with strong visible photoluminescence (PL) and has demonstrated one of the first solid-state electroluminescent (EL) devices in bulk monocrystalline Si material. In addition, we have extended our work beyond bulk Si and have demonstrated visible PL in thin films of porous polycrystalline Si (PPSI) formed on glass substrates. Very recently, we have demonstrated the first electrically induced visible luminescence in PPSI films formed on ITO-coated glass faceplates. Though not measured quantitatively, the EL emission can be viewed clearly through the glass faceplate in daylight. In this paper, we discuss the potential display applications of both bulk and thin-film porous Si and explain some of the challenges involved in fabricating devices using these new materials.
This paper reports the surface electronic structure of light-emitting porous polycrystalline silicon (PPS) using X-ray photoelectron spectroscopy (XPS). We find that the PPS films with strong photoluminescence (PL) effect can only be observed in thin film with trace amount of silicon nanoclusters and the luminescence can be enhanced remarkably with proper passivation of the PPS surface. Incomplete oxidation of silicon (Si3+ or Si2+) does not lead to visible PL. We further estimate that the average size of silicon nanoclusters is in the range of 20–30 Å in the sample having PL emission.
For the formation of PS dielectric filters a detailed calibration of the etch rates and refractive indices is required. The effective dielectric function of PS was determined for different substrate doping levels as a function of the anodization current density by fitting reflectance spectra. Based on these results a number of different dielectric filters were realized. For device applications a thermal oxidation step is necessary to reduce aging effects which occur as a result of the native oxidation of PS. In addition, thermal oxidation results in a qualitatively improve filter performance due to a reduced absorption in the PS layers. Therefore the dielectric functions of PS oxidized in dry O2 at temperatures up to 950 °C were determined. A continuous variation of the porosity and hence the refractive index with depth was used to realize so-called rugate filters. This type of interference filter allows the design of structures with more complex reflectance or transmittance characteristics than structures consisting of discrete single layers. © 1997 Elsevier Science S.A.
We report visible light emission from porous structures formed in bulk and thin‐film polycrystalline silicon materials by anodic etching in an HF:ethanol solution. Our results indicate photoluminescence (PL) peaks at wavelengths between 650 and 655 nm and with intensities comparable to those typically obtained from porous samples of single‐crystal silicon. The analyses of the surface morphology of porous polycrystalline silicon (PPSI) layers suggest that the etch rate could be preferentially greater at the grain boundaries. We have illuminated PPSI films formed on quartz substrates from both the front and rear of the samples and have measured PL emission from the same corresponding sides. Luminescent polycrystalline silicon films offer the possibility of integrating a novel Si‐based flat‐panel display along with the recently developed thin‐film transistor (TFT) driver circuitry on a glass substrate. In addition, nanostructures originating from polycrystalline silicon substrates may enable low‐cost fabrication of highly efficient photovoltaic cells.
A highly sensitive photodetector was made with a metal‐porous silicon junction. The spectral response was measured for the wavelength range from 400 nm to 1.075 μm. It was demonstrated that close to unity quantum efficiency could be obtained in the wavelength range of 630–900 nm without any antireflective coating. The detector response time was about 2 ns with a 9 V reverse bias. The possible mechanisms are discussed.
Optical waveguiding is demonstrated in porous silicon multilayers. Depth variations in porosity, and therefore refractive index, are achieved by switching between high and low current densities during the anodic etch process. Planar waveguiding has been demonstrated at λ = 1.28 μm. The wavelength range has been extended to the visible (λ = 0.6328 μm) by oxidising the samples to produce layered porous oxide structures. Two-dimensional strip-loaded waveguides have been produced, for both the visible and infrared, by etching into each top layer through a pre-deposited photolithographically-defined mask.
Indium and aluminum can be deposited into the pores of porous silicon by an electroplating process. The electroluminescence efficiency is highly increased by this process. The impact of the metals on the luminescence is interpreted by a doping effect of In and Al with respect to the n-type silicon.