Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers
TL;DR: In this paper, free standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography using electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers.
Abstract: 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−2) 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.
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TL;DR: New aspects of derivatives of Si NCs in applications that utilize their optical absorption and emission features are covered, including interfacial chemistry, that are emerging as important elements for increasing the understanding of the effect of quantum confinement in nanostructured Si and for realizing efficient fluorescence emission.
Abstract: The optical use of colloidal silicon nanocrystals (Si NCs) has gained increasing attention for its possible contributions to building a sustainable society that ideally uses resources and energy with high efficiency without causing damage to the environment or human health Si wafers (Eg ≈ 11 eV) dominate modern microelectronics as an impressive electronic material, but they exhibit relatively poor optical performance owing to an indirect bandgap structure Interestingly, however, full control of the size distribution and surface chemistry of the NCs yields size-dependent light emission in a very wide range from near-ultraviolet through visible to near-infrared wavelengths In addition to such unique luminescence properties, Si exhibits a high chemical affinity to covalent linkages with carbon, oxygen, and nitrogen, thereby producing almost unlimited variations in organic–Si NCs architectures hybridized at the molecular level To achieve this goal, I note some parameters, including interfacial chemistry, that are emerging as important elements for increasing our understanding of the effect of quantum confinement in nanostructured Si and for realizing efficient fluorescence emission This article covers new aspects of derivatives of Si NCs in applications that utilize their optical absorption and emission features
75 citations
TL;DR: In this article, a stable blue-green light emission at the peak wavelength down to 500 nm was achieved, and the formation of Si dihydride on the sidewall surfaces of the Si rods is not responsible to the visible luminescence for the very thin Si wires.
Abstract: A boiling water treatment of light emitting porous silicon can give rise to a large blue shift of its photoluminescence spectrum and meanwhile strengthen the skeleton of porous Si by filling up many pores with aqueous oxide. A stable blue‐green light emission at the peak wavelength down to 500 nm is achieved. FTIR measurements show that the formation of Si dihydride on the sidewall surfaces of the Si rods is not responsible to the visible luminescence for the very thin Si wires.
75 citations
TL;DR: In this article, the radiative recombination rate of excitons confined in Si nanocrystals was modified by placing a Au layer nearby, and an experimentally obtained oscillation behavior was compared with a calculated one.
Abstract: The radiative recombination rate of excitons confined in Si nanocrystals was modified by placing a Au layer nearby. Oscillation of the rate was observed when the distance between the active layer and the Au layer was changed. By comparing the experimentally obtained oscillation behavior with a calculated one, the radiative and nonradiative recombination rates, and also the internal quantum efficiency of excitons in Si nanocrystals were estimated. The relation between the radiative rate and the luminescence wavelength was on a single curve for all the samples studied. On the other hand, the nonradiative rate depended strongly on samples. For the samples annealed at $1250\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, the estimated quantum efficiency was close to 100% at longer wavelength side of the luminescence bands, while the maximum quantum efficiency was 70% for the sample annealed at $1200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The present results provide evidence that in Si nanocrystal assemblies, the majority of nanocrystals in samples do not contribute to photoluminescence and a small part of nanocrystals luminesce with high quantum efficiencies, and thus the total quantum efficiency is mainly determined by the number ratio of bright and dark Si nanocrystals in the assembly.
75 citations
TL;DR: In this article, the authors show that film microstructure is one of the main factors, determining long-term photoluminescence (PL) properties, and that films with different porosity exhibit different peak energies, integral intensities and time-dependent evolutions.
Abstract: Pulsed laser ablation in an inert gas has been used to fabricate films containing silicon nanocrystals. We show that film microstructure is one of the main factors, determining long-term photoluminescence (PL) properties. Films with different porosity were found to exhibit PL signals with quite different peak energies, integral intensities and time-dependent evolutions. The distinction of these PL properties is attributed to the different efficiency of surface chemistry interactions between Si nanocrystallites and the ambient atmosphere for films having different porosities. Oxygen-related defects and other mechanisms are discussed to explain the PL properties of the films.
75 citations
TL;DR: In this paper, the authors demonstrate that a porous film of silica nanoparticles emits a bright visible luminescence associated with defects stabilized by oxygen chemisorption at oxygen-deficient center sites.
Abstract: We demonstrate that a porous film of silica nanoparticles emits a bright visible luminescence associated with defects stabilized by oxygen chemisorption at oxygen-deficient center sites. Time-resolved spectra excited by a tunable laser allow us to distinguish the luminescence at 1.99 eV, characteristic of the nonbridging oxygen hole center (NBOHC) (≡Si–O)3Si–O•, and a fast and a slow emission: the first (lifetime τ ≈ 25 ns) is peaked at 2.27 eV with an excitation spectrum centered at 5.5 eV; the second (τ ≈ 7.5 μs) is peaked at 2.41 eV and is excited around 3.2 and 5.2 eV. Reaction in an air atmosphere leads to the disappearance of the NBOHC luminescence and of the fast band, whereas the slow one remains stable. On the basis of the comparison with previous experimental and computational works, we discuss the role of the silanone Si═O and of the dioxasilyrane Si(O2) as the emitting defects.
75 citations
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TL;DR: In this article, the properties of electrolyte-semiconductor barriers are described, with emphasis on germanium, and the use of these barriers in localizing electrolytic etching is discussed.
Abstract: Properties of electrolyte-semiconductor barriers are described, with emphasis on germanium. The use of these barriers in localizing electrolytic etching is discussed. Other localization techniques are mentioned. Electrolytes for etching germanium and silicon are given.
1,039 citations
TL;DR: It is found that a standard, widespread, chemical-preparation method for silicon, oxidation followed by an HF etch, results in a surface which from an electronic point of view is remarkably inactive, which has implications for the ultimate efficiency of silicon solar cells.
Abstract: We have found that a standard, widespread, chemical-preparation method for silicon, oxidation followed by an HF etch, results in a surface which from an electronic point of view is remarkably inactive. With preparation in this manner, the surface-recombination velocity on Si111g is only 0.25 cm/sec, which is the lowest value ever reported for any semiconductor. Multiple-internal-reflection infrared spectroscopy shows that the surface appears to be covered by covalent Si-H bonds, leaving virtually no surface dangling bonds to act as recombinatiuon centers. These results have implications for the ultimate efficiency of silicon solar cells.
910 citations
TL;DR: In this paper, multiple internal infrared reflection spectroscopy has been used to identify the chemical nature of chemically oxidized and subsequently HF stripped silicon surfaces, and these very inert surfaces are found to be almost completely covered by atomic hydrogen.
Abstract: Multiple internal infrared reflection spectroscopy has been used to identify the chemical nature of chemically oxidized and subsequently HF stripped silicon surfaces. These very inert surfaces are found to be almost completely covered by atomic hydrogen. Results using polarized radiation on both flat and stepped Si(111) and Si(100) surfaces reveal the presence of many chemisorption sites (hydrides) that indicate that the surfaces are microscopically rough, although locally ordered. In particular, the HF‐prepared Si(100) surface appears to have little in common with the smooth H‐saturated Si(100) surface prepared in ultrahigh vacuum.
588 citations
TL;DR: In this article, the authors measured hydrogen desorption from monohydride and dihydride species on crystalline-silicon surfaces using transmission Fourier-transform infrared (FTIR) spectroscopy.
Abstract: Hydrogen desorption kinetics from monohydride and dihydride species on crystalline-silicon surfaces were measured using transmission Fourier-transform infrared (FTIR) spectroscopy. The FTIR desorption measurements were performed in situ in an ultrahigh-vacuum chamber using high-surface-area porous-silicon samples. The kinetics for hydrogen desorption from the monohydride and dihydride species was monitored using the SiH stretch mode at 2102 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the ${\mathrm{SiH}}_{2}$ scissors mode at 910 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. Annealing studies revealed that hydrogen from the ${\mathrm{SiH}}_{2}$ species desorbed between 640 and 700 K, whereas hydrogen from the SiH species desorbed between 720 and 800 K. Isothermal studies revealed second-order hydrogen desorption kinetics for both the monohydride and dihydride surface species. Desorption activation barriers of 65 kcal/mol (2.82 eV) and 43 kcal/mol (1.86 eV) were measured for the monohydride and dihydride species, respectively. These desorption activation barriers yield upper limits of 84.6 kcal/mol (3.67 eV) and 73.6 kcal/mol (3.19 eV) for the Si-H chemical bond energies of the SiH and ${\mathrm{SiH}}_{2}$ surface species.
479 citations