Microstructure and formation mechanism of porous silicon
TL;DR: In this paper, the effects of silicon dopant type, resistivity, current density, and hydrofluoric acid concentration on the formation and properties of porous silicon were investigated using cross-section transmission electron microscopy.
Abstract: A systematic study is presented of the effects of silicon dopant type, resistivity, current density, and hydrofluoric acid concentration on the formation and properties of porous silicon. Cross‐section transmission electron microscopy revealed the presence of two distinct microstructures. The structure formed is determined by the doping level with the transition occurring near degeneracy. A model of the anodisation process is presented which is based on the semiconducting properties of the material and which explains the formation of the two different types of porous structure observed.
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01 Aug 1998
TL;DR: In this article, the available etching methods fall into three categories in terms of the state of the etchant: wet, vapor, and plasma, and they are reviewed and compared by comparing the results, cost, complexity, process compatibility, and other factors.
Abstract: Bulk silicon etching techniques, used to selectively remove silicon from substrates, have been broadly applied in the fabrication of micromachined sensors, actuators, and structures. Despite the more recent emergence of higher resolution, surface-micromachining approaches, the majority of currently shipping silicon sensors are made using bulk etching. Particularly in light of newly introduced dry etching methods compatible with complementary metal-oxide-semiconductors, it is unlikely that bulk micromachining will decrease in popularity in the near future. The available etching methods fall into three categories in terms of the state of the etchant: wet, vapor, and plasma. For each category, the available processes are reviewed and compared in terms of etch results, cost, complexity, process compatibility, and a number of other factors. In addition, several example micromachined structures are presented.
780 citations
Cites background from "Microstructure and formation mechan..."
...If HF/H O is used, the etch rate can beincreased without a passivation limit sinceSiO dissolves readily in HF (it should be noted that if a highly concentrated HF solution, deficient in OH ions, is used with an anodic bias, porous silicon can be formed [49], [50]....
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TL;DR: A review of experimental work on freezing and melting in confinement is presented in this paper, where a range of systems, from metal oxide gels to porous glasses to novel nanoporous materials, are discussed.
Abstract: A review of experimental work on freezing and melting in confinement is presented. A range of systems, from metal oxide gels to porous glasses to novel nanoporous materials, is discussed. Features such as melting-point depression, hysteresis between freezing and melting, modifications to bulk solid structure and solid-solid transitions are reviewed for substances such as helium, organic fluids, water and metals. Recent work with well characterized assemblies of cylindrical pores like MCM-41 and graphitic microfibres with slit pores has suggested that the macroscopic picture of melting and freezing breaks down in pores of molecular dimensions. Applications of the surface force apparatus to the study of freezing and melting phenomena in confinement are discussed in some detail. This instrument is unique in allowing the study of conditions in a single pore, without the complications of pore blockage and connectivity effects. The results have confirmed the classical picture of melting-point depression in larger pores, and allowed the direct observation of capillary condensation of solid from vapour. Other results include the measurement of solvation forces across apparently fluid films below the bulk melting point and a solid-like response to shear of films above the bulk melting point. These somewhat contradictory findings highlight the difficulty of using bulk concepts to define the phase state of a substance confined to nanoscale pores.
515 citations
TL;DR: In this paper, the formation and properties of porous silicon formed by anodising silicon under a wide range of conditions were investigated and the currentvoltage characteristics of the silicon-hydrofluoric acid system were presented.
477 citations
TL;DR: In this paper, the spectral distribution of the emitted light was measured−in situ−during the anodic oxidation step, and recorded spectra show a maximum which shifts continuously from red‐orange at the beginning of the process towards the yellow range.
Abstract: Porous silicon/silicon structures under anodic oxidation conditions give rise to an electroluminescence phenomenon in the visible range. Using an optical multichannel analyzer the spectral distribution of the emitted light was measured−in situ−during the anodic oxidation step. Recorded spectra show a maximum which shifts continuously from red‐orange at the beginning of the process towards the yellow range. The visible emission well above the band gap of bulk silicon is attributed to a quantum size effect in the very small size (5–20 A) silicon island which constitutes the porous silicon skeleton. The light emission is interrupted when the current flow stops due to the formation of a continuous oxide layer at the porous silicon/silicon interface.
413 citations
TL;DR: Porous silicon that strongly emits in the visible was analyzed using Raman scattering and it was shown that the local structure of porous silicon is more like a sphere than a rod and has a characteristic diameter of 2.5-3.0 nm as mentioned in this paper.
Abstract: Porous silicon that strongly emits in the visible was analyzed using Raman scattering. The spectrum peaks near 508 cm−1, has a width of ∼40 cm−1, and is very asymmetric. Using a model of phonon confinement, this suggests that the local structure of porous silicon is more like a sphere than a rod and has a characteristic diameter of 2.5–3.0 nm. Polarization Raman measurements suggest that the structure does not consist of a series of parallel columns.
307 citations
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